The new model in cosmology has the universe consisting of about
73% dark energy, 23% dark matter and 4% matter that we can observe. Most think there are two kinds of dark matter, some of it
ordinary (baryonic) matter we don't see because it is not reflecting
light and we can't "observe" it. There is also exotic dark matter
that we don't know exactly how to describe, but I don't think they
believe it has "anti-gravity" properties. Dark energy is the other
stuff, or some call it the cosmological constant, or some call it
quentessence. Nothing is known about it except for one thing.
I'm sure you have heard of the increasing rate of expansion of the
universe. This has been checked by different methods to verify
the universe is indeed expanding at a faster and faster rate and
is generally agreed upon. The problem is, how could this be happening? It should be slowing down this long after the big bang,
and gravity, an attractive force, should be slowing the expansion also.
But instead, it is expanding faster and faster, some of the most distant quasars are even receding from us faster than light. That is
where dark energy has to come into play. Something with repulsive-
gravity properties has to be causing the expansion, gravity tries to
draw things together and something has to be causing the opposite
effect. Nothing is known about it, only the observed effects.
