Why do astronomers hypothesize that a massive black hole lies at the center of m87?

According to Einstein's theory of relativity, a black hole is a "singularity" that consists of a region of the space in which the density of matter tends to infinity. In consequence, this huge massive body has a gravitational pull so strong that not even light can escape from it.

In addition, "the surface" of a black hole is called the event horizon, which is the border of space-time in which the events on one side of it can not affect an observer on the other side.  

In other words, at this border also called "point of no return", nothing can escape (not even light) and no event that occurs within it can be seen from outside.  

In this sense, and according to the relativity, it is possible to determine where a black hole is if it is "observed" an enormous amount of energy released. So, in accordance to this, galaxies like ours must have a black hole in its center.

On the other hand, the elliptical galaxy Mesier 87 (also called Virgo A, but from now on M87) was showing the above described behaviour, with enormous jets of high-energy particles shooting away from its vicinity . This was imaged by the Hubble Space Telescope years ago; that is why astronemers were hypothesizing about the existence of a massive black hole there.

Well now, on April, 10th 2019 this was demonstrated with the publication of the image, for the first time, of the event horizon of the black hole in M87. This is the first time in human history a picture of a black hole is taken.

This was done by the huge effort of diverse scientist and by the syncronization of eight radio telescopes scattered across the Earth (located at: Hawaii, Spain, Chile, Mexico, Arizona and the South Pole), which took the same point of the sky at the same time.

scientific laws are the result of testing theories and proving them to be facts.