Big Bang (noun, “Big Bang”)
This theory explains how our universe began. About 14 billion years ago, all the matter — and energy — that makes up the universe was squashed into an incredibly small point. Because the matter was so condensed, it wasn’t in any form we would recognize today. There were no atoms or even particles. Suddenly, though, it all went through a rapid expansion — an explosion, in a way. That’s the Big Bang. The result was a super-hot, super-dense “cosmic soup” of matter and energy. Over tiny fractions of time, light and charged particles such as protons emerged. Only a few minutes after the Big Bang, the element hydrogen formed. Later, over millions of years, matter began to clump into what would become stars and planets. The universe kept on cooling and expanding, and forming more complex structures, such as galaxies. And it continues to do so today.
In a sentence
The best evidence for the Big Bang is the remnants of the light — called cosmic background radiation — released from the original explosion.