Black holes

Sadly black holes are not portals to other universes. Anyway you can't really see black holes because they are so powerful they suck in light itself, but scientists can record them by studying the effect they have on things around them. Black holes are incredibly dense points in space where the gravity is so strong all the laws of physics you have learned or will learn at school will get thrown out of the window. 

After some stars have been through a supernova, their cores collapse in on themselves so much that they form a single, incredibly dense point in space called a singularity. The space immediately surrounding the singularity is called a black hole. most scientists believe that gravity would crush anything that entered a black hole out of existence.

A cloud of gas or a star passing a black hole within the event horizon (boundary where the black hole's force reaches) will be torn apart by the black holes gravity. As matter is pulled sharply towards the black hole, it speeds up and rubs against other matter, generating friction. This creates incredibly hot temperatures as high as 1 billion degrees . This gives of lots of x rays which can be detected as they travel through space. This is another way to detect black holes.

Finally, and coming to the point, the first black hole to be discovered was Cygnus X-I in 1971. It is thought to be 30 to 60 kilometres in diameter, yet has the mass of as many as 10 suns.