Studies of the motions of stars in clusters and galaxies all over the universe reveal that the motions of visible objects are in response to gravitational forces 10 times greater than other visible objects can account for.  Thus 90% of the mass in all large systems is carried by some unknown individual particles, not by visible stars, gas and dust... that is, not by atoms!  These are called WIMPS or Dark Matter Particles.

Due to gravity, one would expect that as the universe expands each individual galaxy would be slowing down, and if the four fundamental forces were all that exist in nature, this expansion would slow down in a very distinctive way called “flat-space expansion.” But in fact the observed rate of expansion at large distances is increasing! This might seem to suggest that there is another force of nature similar in strength to gravity, but repulsive rather than attractive. Einstein instead introduced a property of empty space that today is called “Dark Energy”...

Einstein in his 1915 theory of gravitation gave space itself a property like that of a compressed sponge... it expands on its own, the "stiffness" controlled by a fudge factor, the "cosmological constant." So the accelerated expansion of the universe that we now observe is generally described in terms of this parameter, rather than in terms of a repulsive 5th force.  The nature of this Dark Energy is currently unknown, but because the universe is mainly empty space, Dark Energy dominates the universe and as it expands becomes a larger and larger component.