: what are blackholes?

Blackholes are highly compressed objects, such that the escape velocity from the surface is greater than the speed of light (this is a very simplified picture). Astronomers have identified two classes of candidates. First, when a large star runs out of fuel, it will collapse under gravity. A typical large star will become a supernova, blowing off a lot of material. If the remnant is more than 1.4 solar masses, it will collapse into a black hole. The second class consists of galactic centers, which can be billions of solar masses in a very small volume. I suggest you read Kip Thorne's "Black Holes and Time Warps" to get a much more complete (and more accurate) description.