Q&A: Dark Matter

Q:
Is it true that 95% of the universe is empty and consists of nothing?

A:
It doesn't seem to be true that 95% of the universe is empty and consisting of nothing, however it does appear that 95% of the universe is invisible to us! Observations of the universe so far make us think it is made up of 3 constituents. First, ordinary matter, protons and neutrons, the molecules that make up human beings, the Earth, and other astrophysical bodies. Second, another type of matter called "dark matter," which seems to behave like ordinary matter in that it clumps up into galaxies and clusters of galaxies, but which we cannot see in any wavelength that we have looked at so far. This includes visible light, X-ray light, gamma ray light, and radio. The third is "dark energy" which does not behave like ordinary matter, we do not see it clump up, but it pervades all of space and varies slowly in time. Dark energy has also been called anti-gravity because its effect is to speed up the expansion of the universe, or appear to. Recent observations show that the expansion of the universe is accelerating, not slowing down or at least slowing down the acceleration as some of us expected.

It seems that 70% of the universe is made of dark energy, 25% of dark matter, and only 5% of ordinary matter like you and me. See http://chandra.harvard.edu/chronicle/0403/dark/index.html

More information describing the different parts of the universe:

On our Chandra web site we have an essay on the dark matter mystery:
http://chandra.harvard.edu/xray_astro/dark_matter.html

And other answers we've given to dark matter questions:
http://chandra.harvard.edu/resources/faq/dmatter/dmatter-main.html