Yes.
Black holes does emit radiation, but they cannot be detected from earth. We use gravitational lensing to "see" the black holes.
Black Holes.
Unknown - As they can't easily be detected, and most of them are expected to have been formed upon the onset of the Big Bang. Astronomers estimate that in our region of the Universe, there are some 100 billion supermassive black holes. But this does not account for the lesser size black holes, of which there is estimate to be a much larger number.
Answer
There are already black holes within the universe
Even though black holes suck through parts of the universe, the universe is inevitably big, and growing so as the universe is being sucked into another dimension by black holes, it is also expanding.
Black Holes' can 'eat' any type of matter in the Universe.
Probably stellar mass black holes
No.
The universe likely contains millions upon millions of black holes.
The black holes may not devour everything since the outward velocity of the matter in the universe may escapethe gravitational pull of the black holes. Stephen Hawkins reckons that even the mass in the black holes would diminish over time, though over trillions and trillions of years.
Primordial black holes are theoretical black holes that could have formed in the early universe. They are thought to be small and have a wide range of masses. If they exist, they could have implications for dark matter, gravitational waves, and the evolution of the universe.