This question already has an answer here:
Recent observations discovered really massive black holes, up to $20-40$ billions Solar masses.
Now, according to an recent study and various computer simulations (I'm sorry, I don't have any reference for this, if not a paragraph on a scientific magazine), in a complete ideal case a black hole would grow about $2$ millions Solar masses in $2$ millions years.
Given that, a $20$ billions Solar masses black hole shouldn't exist because the Universe is just $13.7$ billions years. But ok, the simulation indicated an "ideal case" so one should argue about this before.
In any case, there is something I don't get: how is it possible for a $20-40$ billions Solar masses black hole to exist?
A black hole is created by the collapse of a star (according to some criteria and so on), and stars usually live for billions years. So I do expect that a $30$ billions Solar masses black hole has to be really really old, like it does exist since a "short" time after the Big Bang.
How is that possible? How does huge massive black holes have been created?
Something useful maybe
Abstract of the paper I was referring to
And the complete article (hacked)