ELI5: Where does light go?

Two main things happen to cause far away light to dim: diffusion and absorption. With the exception of lasers (which is light of the same wavelenth), visible light is made of many different wavelengths. When that light travels together, the waves overlap and collide, this causes them to spread out. As the light spreads out (diffuses) it gets weaker because there are fewer photons in any particular area. If you are far enough away from the source, there are so few photons that your eye cannot detect them and you don't see any light.

Along the journey, as photons hit objects (trees, buildings, dust in the air) they release some of their energy into the object as heat and bounce off. But, since the object absorbed some of its energy, the photon bouncing off is fainter than before it hit. So, again, if you're far enough away, the photons that reach you may be too dim for your eye to detect, even if there are a lot of them.

But, a steady source of bright light shining in a vacuum can be seen by your eye even after a journey hundreds of light-years long. That's why we can see stars.