The Galactic Center is the rotational center of the Milky Way. The estimates for its location range from 24 -- 28.4 kilolight-years (7.4 -- 8.7 kiloparsecs) from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. There is strong evidence consistent with the existence of a supermassive black hole at the Galactic Center of the Milky Way.

Based on this passage, what is in the center of the milky way galaxy? Let's think fast. Stream of consciousness:
The relevant information to answer the above question is: There is strong evidence consistent with the existence of a supermassive black hole at the Galactic Center of the Milky Way. So the final answer is a supermassive black hole.