Most galaxies (if not all) have unbelievably immense black
holes at their centers. We can't see them, but instruments
like the spectrograph on Hubble can measure how the black
holes affect the matter surrounding them, which is how we
determine their presence.
colorful "zigzag" at left is the signature of a supermassive black
hole in the center of galaxy M84. The black hole was discovered
using Hubble's imaging spectrograph.