Dark energy is not the same as dark matter. They are called ‘dark’ because we cannot detect them directly using electromagnetism (which includes light), presently our only means, and much about them is unknown and remains speculative. Dark energy (sometimes called ‘vacuum energy’) makes up most of the universe, dark matter much of the rest, and everything else–us and other life and planets and stars and galaxies and the rest of the stuff we can detect–only a fraction, most of which is intergalactic gas in the form of hydrogen and helium (see pie chart).
So, about 95% of the universe is made of stuff we can’t detect or know directly. What do we know?
It turns out that empty space isn’t so empty. Hypothetically, it has something called dark energy. I say hypothetically, because evidence for it is indirect, such as why the expansion of the universe is accelerating (red-shifting, pictured), why the observational universe is flat yet space is curved (besides the curvature from matter and dark matter), and why we see large-scale (mega-galactic?) wave patterns of mass density. Dark energy repels, thus countering gravity’s attractive force.
Evidence for dark energy was discovered in the 1990s. There are two leading models explaining what it is, the standard model, where the Big Bang happened once, and the cyclic model, where the Big Bang happens repeatedly. In the cyclic model, dark energy decays and creates conditions for the next Big Bang. In the standard model the energy doesn’t decay but is part of the cosmological constant.
The existence of dark matter was first proposed in 1932. Since then, physicists have noticed that something massive is missing in the orbital speeds and temperatures of galaxies, in gravitational lensing, in the cosmic microwave background. Something unseen, something dark. The evidence suggests that the universe contains ‘far more matter than that which interacts with electromagnetic radiation.’ Perhaps dark matter interacts only with gravity, not electromagnetism (which includes light).
‘According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today.’ No doubt, this search is being carried out in whole or in part at the Large Hadron Collider (pictured).