One of the things I’ve always been most curious about on the topic of space is: is it possible for something that emits light (like a star) to emit light such as radio or gamma waves but not visible light? If such an object existed, it would be invisible to us, although it could still be detected. But it such an object existed and happened to be traveling on a course dangerously close to our solar system, we might not know to look for it with special instruments because we can’t see it.
Stars are primarily powered by fusion – fusing hydrogen atoms into helium (sometimes other elements are involved, but these are most common). This produces a wide range of light waves, from gamma to radio. Visible light is in between those extremes, so it’s expected to be emitted as well.
The closest thing to a star that doesn’t emit visible light is a black dwarf (pictured above), which has yet to be discovered. Once a white dwarf star runs out of fuel, it theoretically will turn into a black dwarf, emitting no heat and no light. The only way to observe it would be if another object’s emitted light bounced off of it. There are no known black dwarfs; the universe (we believe) has not been around long enough for any white dwarfs to burn through all of their fuel. So, the chances of a massive, invisible object speeding through space towards us (or a similarly invisible approaching alien fleet) are essentially zero.