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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Much of the universe is incredibly hot! The majority of the gas within the milky way is several thousand degrees. Even more substantially, much of the gas in between galaxies (which makes up almost half of all normal matter!) is tens of thousands to millions of degrees. (In some cases, it’s over a trillion!)
The issue is that, while it is hot, the density is so low that *it doesn’t transfer much heat*. Heat is what we feel as hot: my hand burns on a pan because the pan is hotter than my hand and transfers heat to my hand. The pan is very good at transferring heat because I’m grabbing a chunk of metal. The hot space gas, on the other hand, has only a handful of particles in a cubic centimeter. Iron would have almost 10^23 in that same volume, so even though it’s a trivial few hundred degrees, the sheer number and density of particles transfers more heat than the very rarefied, albeit extremely hot, space gas.
The Sun is different because we are actually directly receiving its radiation and it heats up an atmosphere that is much more dense than space.
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