[ELI5] Would something stay hot in the space forever since there is nothing to transfer its heat?
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Every object emits what we call _thermal radiation_. It depends on two things: the temperature, and the color.
**The hotter** something is, **the more it radiates off**, and the larger the energy (what we interpret as “color”!) of the average light particle gets. We see that when heating metal, it starts of radiating infrared light which we cannot see, but clearly feel as radiating warmth. A bit hotter, it starts glowing dimly red, then more brightly so, later on switching to orange, yellow, and white. Heated even more, it would become blue, but only very rarely can one encounter objects glowing like that, as the temperature is extreme. For absurdly hot things, it can even get into the ultraviolet range and beyond.
However, that is not the entire truth and only works as above for a _black_ object. Any object is exactly as good at radiating off light as it is in absorbing it; this is due to physics being _reversible_. **Something will only thermally emit what it can eat, color-wise.** This means that a red object which absorbs non-red light, but reflects red, conversely lacks the ability to glow away red light when hot; but any other color is fine.
In conclusion, a very white object has severe issues radiating thermal energy away. Imperfections in the color usually mean that it still can glow visibly. And “white” is a bit of a misnomer, as we would need to expand that into non-visible colors such as infrared, radio waves, ultraviolet and x-rays. There effectively is nothing that is even close to white for many of those extended colors. Hence there will always be _some_ thermal radiation, and the energy for it gets removed from the temperature.
So, long story short, the answer to your question is: no, **things would always cool down over time** unless they have an internal heat source. The way and speed depends on its temperature and color.
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