The total mass of the Universe, or any component of it can't be defined since the Universe is larger than the observable part that we can see and possibly infinite. We can however talk about the relative amounts of mass in different forms given what we can see. When we do this we find that black holes make up a tiny fraction of the average density of the Universe, much less than 1%. See one of the images on http://star-www.st-and.ac.uk/~spd3/" page for a breakdown of the mass in the Universe.
There is no single observation or piece of evidence, rather the models we have describing the formation and evolution of galaxies and stars, which involve many different types of observations to arrive at those theories, suggest that breakdown in relative contributions to the total density that are summarised in the figure I linked to.
Briefly though, the mass in stellar black holes is a little tricky to measure, since we don't see these very well, but can be estimated via knowledge of the Initial Mass Function (IMF) of star clusters, since only the most massive stars end as stellar black holes. For supermassive black holes (those at the centres of galaxies) we can get an estimate of their masses from seeing either their dynamic effect on matter orbiting near them or through theories suggesting relationships between the black hole mass and the spectrum of the radiation emitted from the active region near the black hole.
Yes, however the prediction is that large (very massive) black holes will have a low flux of Hawking Radiation and smaller (less massive) ones will have a larger flux. This means that in principle, since Hawking Radiation removes energy from the black hole, that all Black Holes will eventually 'evaporate' releasing their energy as radiation. Bear in mind that no one has actually observed Hawking Radiation. It is possible (though I think it is thought to be unlikely) that some very powerful explosions observed mainly through Gamma rays could have been produced by the final stages of an evaporating black hole. Since the rate of Hawking radiation produced increases as the hole gets lighter, the final death throngs of a black hole is though to be a very energetic process.
Bear in mind though that this is still all speculation, and it's thought that other processes are probably more likely to be the cause of Gamma ray bursts, but it's possible that at least some of those observed were due to Hawking radiation.
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