If you took all the stars, planets, and galaxies and combined them, they would make up just four percent of the universe. So what about the other 96 percent? The dark part of space consists of two things: dark energy and dark matter. Neither can be seen, at least not with the technology we have today. So how do scientists know they exist? Scientists figured out dark energy and dark matter are real when they studied the effects of gravity on the objects in space that we can see, such as stars and galaxies.
In 1933, Swiss astronomer Fritz Zwicky studied the Coma galaxy cluster and became the first to suspect that dark matter existed. He compared the galaxy's mass to its gravitational pull and noticed that the numbers weren't adding up. The combined mass of all visible matter wasn't enough to explain the force needed to hold the galaxy together. Zwicky reasoned that another kind of matter had to be at work—what he called dark matter.
The Hubble Space Telescope photographed the Coma cluster of galaxies, and a spiral galaxy.
It wasn't until 40 or so years later that scientists found more evidence that matter existed. In 1970s, American astronomer Vera Rubin studied the Milky Way and other spiral galaxies. She observed that the stars in the galaxies were spinning in surprising ways, moving at a quicker speed that defied the rules of physics. She and other scientists came to believe that dark matter was providing extra gravity that caused the stars to travel so fast.
The Milky Way as seen from Earth and from NASA's Chandra X-ray Observatory
Dark energy, which makes up about 74 percent of the universe, is stretching the universe apart. Scientists have long known that the universe is getting bigger, with many galaxies moving father and father away from each other. Then in 1998, experiment showed that the rate of expansion was speeding up. Astronomers concluded that dark energy was pulling bits of matter apart. Gravity is a force that pulls object closer. Dark energy, then, is gravity's opposite.