If an entity is large and powerful enough, is it possible for it to gain a monopoly advantage and manipulate the rises and falls of the virtual currency market?
It will be difficult, it is like they can but they will not be able to manipulate bitcoin in the market. Only altcoin they can manipulate because the volume is still small compared to bitcoin. The point if you want to manipulate requires a lot of money.
Who says they can't? I'm pretty sure a lot of us are aware that it's not only possible, but it is actually happening. Whales don't only have a lot of alts, they of course have bitcoin as well. And if they want to, with the amount of btc they have, they can very well move the market they way they want to.