Did the Chinese Discover America?

All of Dominican surely believe that Christopher Columbus discovered the New world, but a lately theory indicates that the Chinese were the ones who did it.
According to Menzies's theory, the Chinese arrived on American shores 71 years before Mr. Columbus. He claims that because at that time China was a naval superpower with a strong armada.
Moreover, Gavin Menzies says in his work that China made seven major expeditions with ships which were lengthier than Christopher's ships.
Menzies also says that the Chinese did not colonized America because the emperor in that age lost power to his son, and the new one shut the door to explorations.
To me, that theory does not apply to the reality because is widely believe that the Spanish people were the ones who came to the New world before than any one else. Also, there are clear documents that support the actual work of spanish colonists. But the questions are, why the new Chinese emperor stopped the the expeditions? Do you believe that a country with a strong fleet will back away its success? To me, it doesn't make sense because all of us know that any country will stop its achievement or its influence in a foreign land.