Nonbaryonic Matter?
Ordinary matter of the kind that we see around us is called
baryonic matter because the neutrons and protons that make up
most of its mass belong to a class of particles called baryons.
There are indirect theoretical arguments concerning
the production of light elements in the big bang indicating that
dark matter cannot be ordinary baryonic matter. One example of
nonbaryonic matter is neutrinos, since neutrinos are not baryons
(they belong to a class of particles called leptons.
We shall consider this
argument that dark matter and thus the Universe is mostly nonbaryonic matter when we
discuss the big bang.