The Milky Way is part of the Local Group, a neighborhood about 10 million light years across, consisting of more than 30 galaxies that are gravitationally bound to each other. Aside from our galaxy, the most massive one in this group is Andromeda, which appears to be on course to collide with the Milky Way in about 4 billion years.
Scientists studying galaxies observed that the stars in the outer parts are orbiting the galactic centers just as quickly as the stars further in, a violation of Newton's well-established laws of gravitation. They deduced that something other than the stars and clouds of gas and dust known to comprise galaxies was providing extra gravity – lots of it. They calculated that there must be five times as much of this mysterious dark matter, detectable only by its gravitational pull, as there is of the matter we already knew about.
The Local Group is only one of many, many clusters of galaxies, and they are all moving away from each other as more and more space comes into being between them. This means the universe, itself, is expanding. That discovery is what led to the theory of the Big Bang origin of the universe.
Scientists expected that the gravitational attraction of everything in the universe would put the brakes on the rate of expansion, and eventually the expansion would stop or even reverse. But in the 1990s, scientists discovered that the expansion is actually getting faster. The force responsible for this surprising acceleration was dubbed dark energy. No one is sure what it is, but one possibility is that it is energy contained within the very vacuum of space.
Since matter and energy are equivalent scientists have been able to calculate that whatever dark energy is, it comprises about 68 percent of everything in the universe. Dark matter accounts for another 27 percent, leaving only five percent for protons, neutrons, electrons and photons – in other words, everything we see and understand.