Black Holes 　　Most scientists agree that black holes exist but are nearly impossible to locate.A black hole in the universe is not a solid object,like a planet,but it is shaped like a sphere (球体).Astronomers(天文学家)think that at the center of a black hole there is a single point in space with infinite(无限的)density(稠密).This single point is called a singularity (奇点).If the singularity theory is correct,it means that when a massive star collapses,all the material in it disappears into the singularity.The center of a black hole would not really be a hole at all,but an infinitely dense point.Anything that crosses the black hole is pulled in by its great gravity. 　　Although black holes do exist,they are difficult to observe.These are the reasons. 　　● No light or anything else comes out of black holes.As a result,they are invisible to a telescope. 　　● In astronomical terms,black holes are truly.For example,a black hole formed by the collapse of a giant star would have an horizon(视界)only 18 miles across. 　　● The nearest black holes would be dozens of light years away from Earth.One light year is about 6 trillion(万亿)miles.Even the most powerful telescopes could not pick out an object so small at such a great distance. 　　In 1994 the Hubble Space Telescope provided evidence that black holes exist.There are still answers to be found,however,so black holes remain one of the mysteries of the universe. The Hubble Space Telescope helps scientists to understand the nature of the universe.