Learn Spanish (Latin America): No hay nada detrás del espejo.

Yes, it is a double negative, and in Spanish, they are required! If a negative word like nada (nothing) comes after the verb (hay), you must put no before the verb. Literally, it says "There isn't nothing," but it just means "There is nothing."

Could I say Nada hay detrás del espejo without using no?

Yes, you can! If you put the negative word (nada) before the verb (hay), you do not use no. However, putting no before the verb and nada after it (No hay nada) is a much more natural and common way to phrase this in spoken Spanish.

Why is it del espejo instead of de el espejo?

Whenever the preposition de (of/from) is immediately followed by the masculine word for "the" (el), they mash together to form the contraction del. Because "mirror" is masculine (el espejo), the phrase detrás de (behind) plus el must become detrás del.

Does hay mean "there is" or "there are"?

It means both! Hay is a very flexible word. In this sentence it translates to "there is" because we are talking about "nothing," but you use the exact same word if you want to say "there are" when talking about multiple objects.