Is all of our universe… lit? Can you be hurtling through space and accidentally fly head first into a planet because oops you didn’t have your headlights on?
In: Physics
It depends what you define as bright enough to be visible, and what means you’re using to detect objects around you.
There are rogue planets that travel through the galaxy, and have no star they orbit. These planets could be about the best candidate for the situation you describe, where I imagine you are wondering just how big of an object a human might not visually notice if only looking with the naked eye and no headlights.
Keep in mind however that you’d not see the planet, but you’d definitely see the silhouette it blocks out between you and the starry background. So you would have to be pretty inattentive or moving at a pretty high velocity relative to the planet to not notice it approaching, blocking out more and more of the stars in front of you.
So the candidate for what you’d run into would perhaps be much smaller. Or to run into something bigger, you’d need to find yourself located somewhere where much of the starry background is blocked by cold, dark matter. I have no idea if this is something that occurs commonly in the universe, but I wouldn’t be surprised if some condensing clouds of gas could perhaps cool off (ie emit less light) and begin to compress…?
All of this said, you’d have a very long wait ahead of you if you weren’t actively searching for something to crash into. As other posts have said, there’s a mind boggling amount of distance between objects in space. And since bright things like stars are in the parts of the universe with objects closer together, to get into your pitch black planet scenario, you’d be in a part of the universe with even more mind bogglingly large distances between objects. The chance of hitting something planet-sized is not zero, but it is vanishingly small.
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