yes its called a 51% attack. Fortunately it isnt in the interest of any free market institutions to attempt to do such a thing since it would cause a run and so the bitcoins they were able to generate with that control would have no value. Of course the worry is that a government or large corporation might attempt to do it for the sake of destroying bitcoin as a competitor. Fortunately the cost to do this is roughly proportionate to the general level of interest in bitcoin. So when it was cheap and easy to destroy bitcoin governments didnt bother since almost no one used them and as soon as they became more widely used the cost to destroy bitcoin became proportionately more expensive, this trend should continue at infinitum and with a little luck protect bitcoin into adult hood when it should eventually becomes totally un assailable.