Is this sentence essential or nonessential ? one theory exists that might explain the large number of blind musicians worldwide.

i say essential becuase Essential and Nonessential Adjective Clauses. Remember that an adjective clause is a dependent clause that acts like an adjective in the sentence. This means that it gives more information about a noun or pronoun in the sentence. Sometimes the information given by the adjective clause is essential.

one of the central themes of edward abbey's work has been his staunch opposition to the glen canyon dam on the colorado river that created lake powell, destroying stunning scenery and wildlife habitat, endangering many rare species of fish and animals, and also irreparably destroying many important archaeological sites which provided information about the early inhabitants of the united states.

the part where he says my friends