M87 is not our galaxy. Earth and the other planets we know and love are part of the Milky Way Galaxy, so named because of the dense band of stars that passes through it creating a milky-white colored path. This milky white band of stars is only apparent as such from a certain vantage point. From here on Earth, all of the stars we see in the night sky are actually a part of the Milky Way. As such, our galaxy is a relatively light place.
Black holes are areas of the universe so dense that not even light can penetrate them. Thus, these areas appear as black regions, and appear to literally suck the light from surrounding regions inward. The idea that black holes such everything inward is a bit of science fiction. However, they are literally so dense that they can in essence have their own level of extreme gravity. Once objects get close enough to the black hole they can only go onward into the hole because of this gravitational effect. The point of no return, beyond which light or other objects will go forward into the hole, is called the Event Horizon, so named because any event (light emission or other) that happens beyond this point will not be observable outside the hole. Therefore it is impossible to determine if the event occurred at all.
The black hole in the Milky Way Galaxy is a mere 2 billion solar masses, or 2 billion times the mass of our sun. Some estimates place this at closer to 4 billion. But, even that number pales compared with the black hole in M87. The black hole in M87 is now estimated to be 6.6 billion solar masses.
These super massive black holes are probably formed by merging smaller black holes. Smaller black holes are commonly formed by collapsing stars. Once a star runs out of fuel to burn, it cannot maintain itself and collapses in on itself, succumbing to its own gravity. If the star was dense enough, it will form a stellar black hole as a result of this event.
M87 appears to be the result of hundreds or more mergers of smaller black holes, and could now swallow our whole galaxy. In fact, it could swallow more than 4 of them. But, we are in no danger. The M87 black hole is also more than 50 million light years away from Earth.
In addition to being the largest black hole on record, M87 provides physicists their best chance to study black hole physics, which, by and large, is still only theory at this point.
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