What is exactly is dark matter and dark energy and if you can explain it to me what does it do?

Question

What is exactly is dark matter and dark energy and if you can explain it to me what does it do?

Answer 1

to this date..... they are the biggest force in the universe but we know less of it.

Answer 2

In a nut-shell, dark matter is supposed to make up 97% of the mass of the universe, science has held this view for one hundred years, it is ssupposed to slow down the expansion of the universe, but this has not happened, and this dark matter has never been detected. The universe contiunes to expand so science has come up with a proposal that a form of energy, dark energy, must be the reason behind this. If this mysterious dark energy does exist, science admits that it has no idea of where it came from or how to detect it. It's a sad state of afairs.

Answer 3

In physics and cosmology, dark matter is matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. The observed phenomena which imply the presence of dark matter include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component."

The dark matter component has vastly more mass than the "visible" component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter makes a contribution to dark matter but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much as the marking of early maps with "terra incognita."

In physical cosmology, dark energy is the form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space, are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant, because the change may be extremely slow.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.

Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.

Hot dark matter consists of particles that travel with relativistic velocities. One kind of hot dark matter is known, the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and are therefore very difficult to detect. This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density of dark matter.

Hot dark matter cannot explain how individual galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story.

The Concordance Model requires that, to explain structure in the universe, it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.

At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos). None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model. Many supersymmetric models naturally give rise to stable WIMPs in the form of neutralinos. Heavy, sterile neutrinos exist in extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.

Answer 4

Dark energy and dark matter describe proposed solutions to as yet unresolved gravitational phenomena. So far as we know, the two are distinct.

Dark matter originates from our efforts to explain the observed mismatch between the gravitational mass and the luminous mass of galaxies and clusters of galaxies. The gravitational mass of an object is determined by measuring the velocity and radius of the orbits of its satellites, just as we can measure the mass of the sun using the velocity and radial distance of its planets. The luminous mass is determined by adding up all the light and converting that number to a mass based on our understanding of how stars shine. This mass-to-light comparison indicates that luminous matter comprises less than 1 percent of the total mass in the universe.

There is certainly more matter in our galaxy and other galaxies that we cannot see, but other evidence indicates that there is an upper limit to the total amount of normal matter present in the universe. By normal matter, I mean stuff made out of atoms. Recently, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) satellite made precision measurements of the imprint of sound waves on the cosmic microwave background, produced some 400,000 years after the big bang. Because sound propagation depends on the properties of the medium--as anyone who has played with a helium balloon knows--the pattern of the sound waves viewed by WMAP is an indicator of the abundance of hydrogen and helium in the universe. (All other elements were built from these basic building blocks.) These and other results agree with the theoretical predictions of the primordial abundances of the light elements as a result of the nuclear processes that took place in the first three minutes of the universe--also known as big bang nucleosynthesis. Ultimately, very strong arguments have been made that at most 5 percent of the mass-energy density of the universe, and 20 percent of the mass of clusters, is in the form of atoms.

What could dark matter be? Many physicists and astronomers think dark matter is probably a new particle that so far has eluded detection during particle accelerator experiments or discovery among cosmic rays. In order for a new particle to behave as dark matter, it must be heavy (probably heavier than a neutron) and weakly interacting with normal matter so that it does not easily lead to light-producing reactions. The prototypical dark matter candidate particle is something like a neutrino, though all known types of neutrinos are too light and too rare to explain dark matter.

How does dark matter affect the universe? The dark matter problem can also be viewed as a question of the nature of clustering matter. Dark matter must be the basic building block of the largest structures in the universe: galaxies and clusters. Without dark matter, the universe would be a very different place, according to current theories.

And dark matter is not just for explaining the behavior of distant bodies in the cosmos, but is abundant within our galaxy as well. It is estimated that our solar system is passing through a fine sea of dark matter particles with a density as high as roughly 105 per cubic meter and a velocity of roughly 200 kilometers per second. We may hope to detect the flux of dark matter passing though the Earth, and even to detect the seasons of dark matter, corresponding to the times of year when the Earth is moving with, or against, the flow of dark matter orbiting the center of the Milky Way.

Dark energy, on the other hand, originates from our efforts to understand the observed accelerated expansion of the universe. In a nutshell, current theory cannot explain the acceleration. One speculative possibility is that the acceleration is a consequence of another new form of matter, nicknamed dark energy, which has hitherto gone undetected. It is called "dark" because it must necessarily be very weakly interacting with regular matter--much like dark matter--and it is referred to as energy because one of the few things we are certain of is that it contributes nearly 70 percent of the total energy of the universe. If we can figure out what it really is, it is certain we will find a more illuminating name.

With the establishment of the big bang cosmological model, it had widely been expected that since the birth of the universe some 13.7 billion years ago, the cosmic expansion had been slowing down. But two independent research teams found in 1998 that the expansion was speeding up. If you consider that this expansion is the single most remarkable property of the universe as a whole, then the discovery of the acceleration is truly a breakthrough.

The acceleration is determined by measuring the relative sizes of the universe at different times. Specifically, astronomers measure the redshifted spectra of, and luminosity distances to, stellar explosions called type 1a supernovae. The time required for light from a supernova to reach our telescopes is encoded in the distance (the relation is slightly more complicated than distance = rate x time, due to the cosmic expansion), while the change in the size of the universe from explosion to observation stretches the wavelength of the emitted light, as characterized by the redshift. A comparison of these sizes at a sequence of times reveals that the universe is growing at an ever faster rate. Since this discovery measurements have improved and other cosmological phenomena, also sensitive to the rate of expansion, have been used to confirm these results. (One note: the expansion rate and the acceleration are not measured in meters per second and m/s2, respectively. Rather, they measure the rate of change in the dimensionless scale of the universe, and the second derivative thereof, so the units are 1/s and 1/s2.)

Einstein's theory of general relativity predicts that the cosmic acceleration is determined by the average energy density and pressure of all forms of matter and energy in the universe. Yet no known forms of matter can account for acceleration. Thus, something other than dark matter, atoms, light, etc., must be responsible. One leading hypothesis is that the universe is filled by a uniform sea of quantum zero point energy, which exerts a negative pressure, like a tension, causing spacetime to gravitationally repel itself. This stuff, sometimes referred to as a cosmological constant, was first introduced by Einstein in another context (something he later referred to as his greatest blunder), but that's another story.

How is dark energy affecting the universe today? It is responsible for the cosmic speeding, and international teams of astronomers are working to refine measurements of that acceleration. At stake is judgment on Einstein's greatest blunder (the cosmological constant), possible insight into the fundamental theory of nature (quantum gravity and the quantum state of the universe), and the fate of the universe (a Big Chill or a Big Rip?).

It is tempting to try to combine the explanations for dark matter and dark energy, but there are great differences between the two. Dark matter pulls and dark energy pushes. That is, dark matter is invoked to explain greater-than-expected gravitational attraction. In contrast, dark energy is invoked to explain weaker-than-expected, and in fact negative, gravitational attraction. Furthermore, the effects of dark matter are manifest on length scales roughly 10 megaparsecs and smaller, whereas dark energy appears only to be relevant on scales of roughly 1,000 megaparsecs or greater. Finally, it is important to question whether the dark matter and dark energy phenomena may have gravitational explanations. Perhaps the laws of gravitation differ from Einstein's theory. This is certainly a possibility, but so far general relativity has not failed a single test. And striking new views of clusters have revealed behavior that is inconsistent with a gravitational cure--meaning that dark matter really is there. We are left looking for new particles and fields to fill in the missing matter and energy.