Why there is vaccum in space?

Answer 1

The vacuum in space can be attributed to several principles in physics; properties of matter, the forces that govern them, and properties of the universe. Observations of the physical universe indicate that matter tends to clump, or in other words be unevenly distributed. This can be attributed to the force of gravity, which causes particles with the property of mass to attract to each other; because gravity is effective over immense distances it is largely the cause of the gross structures of matter which accumulates into stars, galaxies, etc. Once a mass is present, its gravitational field will tend to attract other particles in the area, and with no other force to inhibit their movement, they will accumulate into the mass and leave a vacuum behind.
The outward expansion of the universe after the big bang and the expansion of space itself both tend to decrease the density of the universe as a whole (the ratio of matter to the space it's contained in). There also appear to be forces which tend to increase the rate of outward expansion of the universe, accelerating the universe's expansion.
The vacuum of space itself is not total; within the galaxy it is expected several atoms of hydrogen to be present in each cubic meter; in intergalactic space something around maybe a single atom on average per cubic meter.