Einstein realized that if gravity were the only force acting in the universe, it would necessarily cause matter to congeal and spacetime to contract in on itself. Yet since such a contraction hasn’t happened (at least not yet) and since, further, the universe we observe today contains matter surrounded by empty space, Einstein thought something—some outward-pushing force of expansion—must be counteracting the effect of gravitation to account for the empty space between massive bodies in the universe.
