|Universe Dark Energy-1 Expanding Universe|
This diagram reveals changes in the rate of expansion since
the universe's birth 15 billion years ago.
The more shallow the curve, the faster the rate of expansion.
The curve changes noticeably about 7.5 billion years ago,
when objects in the universe began flying apart as a faster rate.
Astronomers theorize that the faster expansion rate is due to a
mysterious, dark force that is pulling galaxies apart.
About one-quarter of the cosmos is comprised of dark matter — five times that of the ordinary matter that makes up everything we see. Dark matter is often defined by what it isn't: something that can be seen and something that is energy.
Scientists are pretty sure dark matter exists, but they are not certain what it is made of or how it interacts with ordinary matter. It is considered vital to all the scientific theories explaining how the universe is expanding and how galaxies move and interact. : NPR
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 68% of the Universe is dark energy. Dark matter makes up about 27%. 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. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.