Outer space is often described as a vacuum, but there is always some matter present. In the space 400 km above Earth's surface, there is as little as 1.58 x 10-14 g of matter in a 500.0 cm3 volume of space. Based on this data, what is the density of the matter in space?

The seven sisters, seven stars located more than 400 light-years away in the taurus constellation can be seen here with venus and another constellation, orion. the seven stars stay grouped together but seem to move, like the planet venus. why do stars seem to move like planets in the night sky?