Because the universe is expanding. Space itself expands the further away you are from a reference point. That means, everything is moving away from us (on a grand scale), and the greater the distance to the reference point, the bigger is the rate things in the universe are moving away from us. I think the rate is about 70 km/(Mpcs) - wich is an incredibly small number, scaled down to a meter it is ~10^-23 m / (ms). And as space expands over time, the density of matter in space get lower and lower.