Unified field theory
A single framework uniting all forces of nature — Einstein's unfinished final quest.
A unified field theory seeks to describe all the fundamental forces and particles of nature within one consistent framework. Einstein spent his last three decades pursuing one that would merge gravity with electromagnetism.
He never succeeded, and worked largely apart from mainstream physics, but the dream of unification still drives theoretical physics today.
Source: Wikipedia: Unified field theory — Read the full article on Wikipedia.