The sun warms up our planet but space is cold. If I lit a candle and stood 20 feet away, I could see the light but not feel the warmth. So, why do we feel warmth from the sun but space isn’t warmed by it?
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**Pretty much every comment is absolutely wrong.** The temperature of space is around 2.7 degrees kelvin (2.7c above absolute zero) which can be measured by looking at the cosmic microwave background radiation, which is light that was released as soon as the universe because transparent to photons.
This light is pretty much perfectly uniform in any direction you look, so even if you have a region of space with “nothing” in it, there is still microwave light from the big bang floating around.
**Why is it cold?** Because that light has been stretched over the past ~14 billion years. When the light was originally created, it was extremely energetic, but as the universe has expanded in size, the light has been stretched into microwaves. It is the same effect that causes sirens to change in pitch as the move towards/away from you.
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