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