Why is it cold in space?

The average temperature in deep space is about 3 degrees above absolute zero(that's the coldest anything can get).But some parts of space are hotter.Near a star,space is hot because stars give off radiation.Earth is swarm because it's close to the Sun,and it's atmosphere trap's the Sun's heat.But most of space is very cold because the universe is always expanding.The Big Bang theory explains how the universe was created with a huge explosion where matter started moving outward.As matter expanded,it good cooler.Scientists can measure this temperature and call it the cosmic backgroud radiation.It's the same everywhere in the universe,unless a star like the Sun is around to heat things up.



reference from national geographic channel