Flux is the presence of a force field in a specified physical medium, or the flow of energy through a surface. In electronics, the term applies to any electrostatic field and any magnetic field . Flux is depicted as "lines" in a plane that contains or intersects electric charge poles or magnetic poles.
Just look it up
The main reason for this is gravity. Gravity between objects in our solar system and milky way is strong enough to counteract this expansion. Hope it helps.