4. Vacuum Energy
Empty space is not really empty, it is full of bubbling virtual subatomic particles that are created and destroyed almost immediately. Based on conservation principle, these particles can’t be created from nothing and since mass is equivalent to energy, there should be a form of energy that hides somewhere in the emptiness of space. Physicists call it “vacuum energy” and if it does exist, vacuum energy should be detected by its gravitational force. Apparently, the vacuum energy concept seems to be confirmed by the current expansion of the universe.