Echo, a repeated sound. The term is usually used to refer to sound that is reflected from a surface and returned to its source after a noticeable delay. A person can typically hear his or her own echo where a hard surface, such as the side of cliff, reflects sound waves directly back to the person. The use of echoes to locate objects is called echolocation.
You've heard of the big bang, of course, but do you have any idea as to what was happening during that massive flurry of activity billions of years ago?
Every day, astronomers unravel a little more of the universe's inner workings, but the jury is still out on 95 percent of its contents.