What's the opposite of major depressive disorder?
There are two ways to interpret/answer this question. First, is there a recognized disorder that is characterized by mania in the absence of depression? Second, are there people who experience mania in the absence of depression?

The answer to the first interpretation is no. There is currently no "major manic disorder" or "unipolar mania." Given the DSM-5 criteria, a person with mania in the absence of depression would likely receive a diagnosis of Bipolar I Disorder, which is the same diagnosis that they would get if they had mania with depression.

The answer to the second interpretation is obviously more difficult. I don't have concrete data, but it appears to be rare. See the following link for an interesting discussion. 

Why no unipolar mania listing in DSM-IV?