Composition of dark matter: baryonic vs. nonbaryonic Dark matter

dark matter can refer substance interacts predominantly via gravity visible matter (e.g. stars , planets). hence in principle need not composed of new type of fundamental particle could, @ least in part, made of standard baryonic matter, such protons or electrons. however, reasons outlined below, scientists consider dark matter dominated non-baryonic component, composed of unknown fundamental particle (or similar exotic state).

fermi-lat observations of dwarf galaxies provide new insights on dark matter.


baryonic matter

baryons (protons , neutrons) make ordinary stars , planets. however, baryonic matter encompasses less common black holes, neutron stars, faint old white dwarfs , brown dwarfs, collectively known massive compact halo objects (machos). ordinary, hard see, matter explain dark matter.

however multiple lines of evidence suggest majority of dark matter not made of baryons:

sufficient diffuse, baryonic gas or dust visible when backlit stars.
the theory of big bang nucleosynthesis predicts observed abundance of chemical elements. if there more baryons, there should more helium, lithium , heavier elements synthesized during big bang. agreement observed abundances requires baryonic matter makes between 4–5% of universe s critical density. in contrast, large-scale structure , other observations indicate total matter density 30% of critical density.
astronomical searches gravitational microlensing in milky way found @ small fraction of dark matter may in dark, compact, conventional objects (machos, etc.); excluded range of object masses half earth s mass 30 solar masses, covers plausible candidates.
detailed analysis of small irregularities (anisotropies) in cosmic microwave background. observations wmap , planck indicate around five-sixths of total matter in form interacts ordinary matter or photons through gravitational effects.

non-baryonic matter

candidates nonbaryonic dark matter hypothetical particles such axions, sterile neutrinos or wimps (e.g. supersymmetric particles). 3 neutrino types observed indeed abundant, , “dark”, , matter, because individual masses – uncertain may – tiny, can supply small fraction of dark matter, due limits derived large-scale structure , high-redshift galaxies.

unlike baryonic matter, nonbaryonic matter did not contribute formation of elements in universe ( big bang nucleosynthesis ) , presence revealed via gravitational effects. in addition, if particles of composed supersymmetric, can undergo annihilation interactions themselves, possibly resulting in observable by-products such gamma rays , neutrinos ( indirect detection ).