Depends on how you define darkness. Scientifically, dark is the absence of electromagnetic energy. A perfectly dark object would not radiate any energy, and would thus have to be absolute zero. The latter is only theoretically possible. It's called heat death, a potential cause of the end of the universe. The point where the universe will expand and cool so much that all energy radiation stops.
Either that, or it would have to have such a high gravity that light cannot escape it, like a black hole. Mind you, quantum mechanics suggests that they do emit radiation, so are not truly completely dark. That's where things get complicated.
That's the long winded explanation for you.