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?
Those entities are known as whales, some individuals and exchanges have an amount of bitcoin so big that they are able to manipulate the market, however they cannot manipulate it fully, they need to wait for the right opportunity, for example when segwit was activated a bull run was inevitable but the whales were very smart and fueled it and we reached very high prices as a result of their actions.