Black hole
If u have heard only one thing about black hole, it's probably that, once it is in the black hole's event horizon, nothing escapes not even light can capable of that. 
At which point is natural to wonder,if nothing can escape a black hole,how could we observe them? How could we even know it exists? 
Well,only things in the black hole's event horizon are stuck - black hole's also gravitationally pull stuff that are outside their event horizon and by that stuff we can come to a really good sense that a black hole is nearby.
For example, lots and lots are orbit in pairs,but we also see stars orbiting thing that are not normal stars.But instead they emit crazy amounts of X- rays and X rays in space often come from dust and gas that gets superheated while spiralling into very dense and very heavy object. 
Anyway by figuring out the mass and orbital characteristics of the stars whose partners emit X rays we can determine how heavy their partners are.Some partners are light enough to become neutron stars but neutron star can only get so big before they collapse in on themselves - theoretical calculations put their upper size limit is about 2-3 times the mass of the sun and the biggest one we observed all fall inside that limit.
And yet there are plenty of stars whose orbits clearly show that their partners are 5- 10 times the mass of the sun and we simply don't know anything could be other than the black hole.
There are lots of objects at the center of the galaxies including our own that emit lots of X- rays,radio waves and infrared radiation but not that much visible light and we know that these objects are stupendously heavy because of the nearby stars and hot glowing dust orbit them.
These orbits tell the objects are so small and so heavy they can't possibly be a star or a cluster of star 
. But the only thing they could be supermassive black holes. 
For example,in the middle of the milky way galaxy, there is a X rays infrared waves emitting object called Sagittarius A with nearby stars orbiting in a such a small fast orbits that we know that they weigh 4 million times the sun 
. So we come to a conclusion these black hole like objects are none other than blackhole only.
Image courtesy : @lokeshlgl
**Thanks for reading** Let the # firepower be with u😎
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