A small proportion of dark matter may be
baryonic dark matter: astronomical bodies, such as
massive compact halo objects, that are composed of ordinary
matter but which emit little or no electromagnetic radiation.
Study of nucleosynthesis in the Big Bang produces an upper bound on the amount of baryonic matter in the universe,[9] which indicates that the vast majority of dark matter in the universe cannot be baryons, and thus does not form atoms. It also cannot interact with ordinary matter via
electromagnetic forces; in particular, dark matter particles do not carry any
electric charge. The nonbaryonic dark matter includes
neutrinos, and possibly hypothetical entities such as
axions, or
supersymmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the
elements in the early universe ("
Big Bang nucleosynthesis")
[3] and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo
annihilation interactions with themselves resulting in observable by-products such as
photons and neutrinos ("indirect detection").
[10]