How dense is dark matter?

Answer 1

This can be answered in two steps:

1) What is the average density of all gravitational matter in the Universe?

It has surprised astrophysicists that this density is extremely close to the "critical" density; above which our Universe eventually collapses in itself, and below which our Universe expands forever.

This critical density is about 5 x 10^-30 grams per cubic centimeter.

2) How much of the mass of the Universe is NOT dark matter?

What shocked astrophysicists even more was determining that the average density of all luminous matter was only about 20% of the critical density. Thus, about 80% of the measured density of our Universe can be accounted for via stuff that gives off light.

The ratio of hydrogen to helium to deuterium in our Universe puts an upper limit on the density of baryonic matter in our Universe. This calculation also shows that the density of baryonic matter can not be more than about 20% of the critical density.

These two, completely independent calculations of the amount of luminous and of baryonic matter -- the stuff we understand -- in our Universe have left scientists with little choice but to conclude that 80% of the gravitational mass in our Universe is composed of something we don't understand. And that's why we call it "dark" matter.

Since the Universe has a density of 5 x 10^-30 grams per cc, and only 80% of that is dark matter, then the latter's average density is about 4 x 10^-30 grams per cc.