The four possible types of matter are known
as cold dark matter, warm dark matter, hot dark matter, and Baryonic matter.
But the most common view is that dark matter is not Baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS(Weakly
Interacting Massive Particles).
This low brightness is the result of the relatively small number of stars within them- most of the normal,
or"Baryonic," matter in these galaxies exists in the form
of huge clouds of gas and dust.