In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together.
In 1998 the Hubble Space Telescope observations of very distant supernovae showed that the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
A new theory has been developed would produce some kind of a field that
would cause this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called
What is "Dark Energy"
More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe.
In astrophysics and cosmology, dark matter is matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter.
Researchers have determined that a major portion, (about 23 percent) of the universe consists of a new kind of matter. No one has seen this matter, and no one knows what it consists of.
Of the remaining 77% of the universe, roughly 73 percent is made up of
something even more enigmatic, called dark energy. Only about 4% is
made up of the things that we are all familiar with.