A hundred years ago, Albert Einstein came up with a theory he thought was so abstract, scientists would never be able to prove it.
But just this month, scientists announced they had.
They found proof of gravitational waves: ripples in space time caused by two black holes slamming together.
The discovery didn't come easily; it took 1,000 scientists from 16 countries 25 years and over $1 billion.
Marc Kamionkowski, a physicist and professor at Johns Hopkins University, put the issue in focus. Watch the video above for more,
