Dark matter is not composed of ordinary baryonic matter, nor is it made of known exotic neutrinos.

Baryonic matter refers to the ordinary matter we interact with daily – protons, neutrons, and electrons – which form atoms. Cosmological calculations based on Big Bang nucleosynthesis (BBN), the process by which light elements like hydrogen, helium, and lithium were formed in the early universe, precisely predict the abundance of these elements. These predictions align perfectly with observations if baryonic matter constitutes only about 4-5% of the total mass-energy of the universe. This discrepancy means the 'missing mass' cannot be baryonic. Furthermore, dark matter does not clump into dark clouds of gas or burnt-out stars (MACHOs - Massive Astrophysical Compact Halo Objects) in sufficient quantities to explain the observations. While neutrinos are known elementary particles that interact only weakly, their mass is too small, and they move too fast (making them 'hot dark matter') to explain the observed large-scale structure of the universe. This necessitates a new type of particle, distinct from anything in the Standard Model, that is 'cold' (slow-moving) and interacts primarily through gravity and possibly the weak nuclear force.