Hi there,'
the force that  acts against the force that keeps the universe expanding is called dark energy.this energy comes from the concept of nothingness that means even vacuum exerts a force on the universe.gravity is the force exerted by earth to objects
i hope this helps u.. 
The Brainliest Answer!

dark matter is a matter that cannot be seen with the help of telescope but accounts as the most in the universe

dark energy is some kind of energy which is said to exist but scientists know a very little about it. about 14 to 15 billion years ago when the big bang happened the universe came into existence. the matter formed from huge energy started expanding due to outward force of the big bang. from the theory of gravity postulated by einstein the particles having mass should attract each other and the expansion of the universe should stop at one point of time. 
however from the observations made by astronomers it appeared that the rate of expansion has increased  after 7 billion years. this shows taht some other force is acting against gravity which is forcing the expansion of universe instead of contraction. scientists gave name to this unknown force as the dark energy

gravity is a phenomenon by which objects attract each other

just don't mind the second one is a  bit  too long

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thank u for marking mine as the brainliest answer