Yes, absolutely - black holes don’t need singularities inside them in order to be black holes. Just enough mass in a small enough volume.
(Of course, we don’t actually know whether stellar or galactic centre black holes have singularities inside them - we just don’t know of any process that will prevent one from forming in current physics.)
> black holes don’t need singularities inside them in order to be black holes.
They do according to the standard GR model. Speculations about holes not having singularities inside them are quantum gravity speculations and we have no way of testing them experimentally any time soon.
No, just inflation taking the rest of the universe over the horizon - I believe it’s the dynamic effects of being in an inflationary universe that mean that the standard GR results for black holes don’t apply, at least according to the references I read. (I can’t pretend to be able to derive this stuff personally.)
> so there's a possible that the visible universe is a black hole?
No. The universe is expanding. If it were a black hole, it would not be expanding. The calculation of how much mass has to be inside what radius in order to form a black hole assumes that the matter is static (or collapsing); if it's expanding, the calculation is meaningless.
