The Ming Dynasty had, reportedly, one of the largest navies in the world during the 15th and 16th centuries, which is when the "New World" was being "discovered" by Europeans. They also had made contact with European traders during this time, and had surely heard about the Americas that were being visited by the Spanish, English, French, etc.
Why did they never send any fleets across the Pacific and try to claim a piece of the "new" landmass that was being explored during this time?