The CMB is the cosmic microwave background, essentially the earliest light in the universe from when atoms are formed. The power spectrum of the temperature flu

The CMB is the cosmic microwave background, essentially the earliest light in the universe from when atoms are formed. The power spectrum of the temperature fluctuations in the CMB contains information about the constituents of the universe because each constituent (radiation, baryonic matter, non-baryonic matter) behaves differently in the early universe. Here is a Scientific American article that offers some explanation: http://background.uchicago.edu/~whu/SciAm/sym1.html http://background.uchicago.edu/~whu/SciAm/sym1.html Microlensing involves gravity. The difference is that normal matter that could be dark matter would be relatively things massive but compact things like black holes and brown dwarfs as opposed to a diffuse halo. The compactness would make them detectable by lensing.