If "expose" means to reveal or make something known:
* Conceal: To hide or keep secret.
* Obscure: To make something unclear or difficult to understand.
* Suppress: To stop something from being expressed or published.
* Protect: To shield something from harm or danger.
If "expose" means to subject someone or something to something:
* Shield: To protect from something harmful or unpleasant.
* Insulate: To protect from external influences.
* Protect: To keep safe from harm or danger.
* Conceal: To hide from view.
If "expose" means to leave something uncovered:
* Cover: To place something over something else to hide or protect it.
* Conceal: To hide from view.
* Encase: To enclose something completely.
The best opposite will depend on the specific meaning of "expose" in the context.