Final answer:
We believe there is a black hole in the Galactic center based on observational evidence. Indirect observations of stars orbiting near the center provide strong evidence for the presence of a black hole with a mass of approximately 4 million solar masses. The alternative explanation, a concentrated region of stars, would eventually collapse into a black hole.
Step-by-step explanation:
We believe there is a black hole in the Galactic center because of the observational evidence we have gathered. Astronomers have found that the very center of our Galaxy contains a black hole with a mass equivalent to 4.6 million Suns. This black hole is called a supermassive black hole because it contains far more mass than the typical black hole created by the death of a single star.
We cannot directly see a black hole because it radiates no energy and the interstellar dust in the center of the Galaxy absorbs visible light. However, we have indirect evidence of the black hole's existence through the observations of stars orbiting near the galactic center. The orbits of these stars provide strong evidence for the presence of a black hole with a mass of approximately 4 million solar masses.
The alternative explanation for the mass in the Galactic center would be stars. But to explain the observed mass without a black hole, we would need to put at least a million stars in a region the size of the solar system. Such a crowded concentration of stars would lead to frequent collisions and mergers, eventually collapsing into a black hole.