Cosmic Scale

The finding suggests that the invisible substance called dark matter and the even more mysterious force known as dark energy are not just figments of physicists' imaginations.

For centuries Isaac Newton's law of universal gravitation worked well enough to explain gravity on Earth. But astronomers eventually saw discrepancies in the way larger objects such as planets interacted.

Einstein's general theory of relativity, published in 1916, proposed that gravity works on large scales because matter warps the fabric of space and time, also known as space-time. (See "Einstein and Beyond" in National Geographic magazine.)

This notion has been used to successfully explain phenomena in our solar system, such as the slight alterations in Mercury's orbit around the sun, which Newton's gravity couldn't account for.

The existence of dark matter and dark energy is based on the assumption that Einstein's gravity is affecting galaxies billions of light-years from Earth in the same way that it affects objects in our solar system.

Based on general relativity, for example, scientists think dark matter exists because some cosmic objects behave as if they have more mass than we can see.

But until now, tests of general relativity on galactic scales have been inconclusive.