Why does dark matter have to be something that doesnt emit EM radiation?

Question

Why does dark matter have to be something that doesnt emit EM radiation?

Answer 1

its supposed to absorb all of it

Answer 2

Dark matter has been theoretically posited as the main 'matter' gravitational 'element' within galaxies. If dark matter emitted any form of EM radiation, or interacted with it, we would be able to detect its presence due to its dominant presence. However, dark matter may not be an exotic form of matter at all!

Some theorists have speculated that dark matter could be neutrinos. Neutrinos have a very small nuclear weak force cross-section (probability of interaction) and it would take a light-year of lead to block half of the neutrinos produced by the sun. Thus, neutrinos become possible candidates for dark matter due to their low interaction rate with matter. Furthermore, neutrinos travel at either, the speed of light or close to it. In the later speed case, they may have a very small mass of less than 2.2  electron-volts (ev for short. To convert to a mass - multiply by the charge on an electron (e) and divide by c^2 (from E = mc^2)). Hence, with this tiny mass, neutrinos may interact gravitationally with the universe. Evidence from solar neutrino experiments suggests that neutrinos exhibit flavour oscillations during their passage. The flavour oscillations result in an electron neutrino transforming into the muon and then the tau neutrino and then back again. The muon neutrino has a mass of less than 170 kev and the tau neutrino has a mass of less than 15.5 Mev. Thus, flavour oscillations could result in neutrinos having a greater gravitational influence. There is, however, one drawback to this theory - neutrinos move at close to the speed of light and so localised gravitational condensation into proto-galaxies is difficult to account for!

Answer 3

Don't mean this to be saracastic, so please don't take it that way, but if dark matter emitted EM radiation, it wouldn't be dark.