The SMBH at the heart of M87 has an estimated ~6 billion times the Sun’s mass and it is accreting material at a prodigious rate. Measurements of the flux from the SMBH show it is at least an order of magnitude below what is expected if a physical surface were present. This indicates that the central compact object in M87 is a supermassive black hole with an event horizon and not some very massive exotic object with a physical surface. Once accreted material falls through the event horizon of a black hole, it is no longer visible to the outside universe.
Broderick et al. (2015), “The Event Horizon of M87”, arXiv:1503.03873 [astro-ph.HE]