The black hole is in our universe therefore the matter that enters stays in our universe.
The evidence of that is that the mass that enters the black hole continues to participate in the gravitational effect of the black hole and that gravitational effect affects our universe (therefore, it does not leave).
The event horizon represents the limit of our interaction with the "object" from our frame of reference. Any information from an object that is "inside" the event horizon will never reach us.
The same way that information about the part of the universe that is beyond the limit of the Visible Universe (the portion we can see) will never reach us -- in that last case, we are inside the horizon and we can't get information about the outside.
We do not "know" what happens to matter that enters the event horizon, going into a black hole, but we can apply the knowledge we have about how particles and sub-particles (and sub-sub-...) to try and understand what happens.
There is one snag: we are not sure we fully understand how time really works inside a black hole. As far as we can tell (but we do not know for sure) and object falling into the black hole keeps accelerating forever, but never reaches the centre.
That is because space is expanding faster and faster (we think) as one tries to approach the singularity.
What we do know is that the gravitational gradient (a.k.a. tidal force) gets very steep, especially for smaller black holes, so that even atoms are ripped apart by the tidal tension (one side of the atom gets a lot more gravity than the other), and even the particles themselves (e.g., a proton) can get ripped apart. When that happens (outside the event horizon), the ripping apart causes very high-energy photons to be emitted (X-rays, gamma rays). They represent the force that tried to keep the particle in one piece... until it got ripped apart.
If the gradient is smooth enough for particles to survive, then they can enter the event horizon before being ripped apart. However, we think that they would eventually be ripped apart to pure energy (but we would never see the resulting photons, as they can't escape back to our side of the horizon).