Because primordial baryonic density fluctuations would have to be extremely large for the vast majority of galaxies we observe in the cosmos to emerge without dark matter, the adiabaticity of CMB fluctuations actually leads to dark matter.
The smooth early Universe and the bumpy older Universe are connected by the CMB. The CMB is fairly homogeneous overall, yet there are small temperature differences or anisotropy. These temperature variations match changes in the universe's matter density at the time the CMB formed.
Up to 90% of the matter in the cosmos may be dark matter or invisible matter. Many astronomers and physicists have proposed a number of explanations for this dark matter. It might just be common material, like dust strewn over the universe, frigid gas, big or small black holes, or ultra-faint stars that produce or reflect too little radiation for our equipment to detect. It might possibly be made up of strange, exotic particles that we are unable to perceive.
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