Some of the most mysterious objects in our galaxy are also among the most numerous. And it turns out that there's an estimated 100 billion mysterious brown dwarfs scattered among the stars.
They are so ubiquitous that there could be one closer to the Earth than the nearest star system, Alpha Centauri. If a brown dwarf is ever discovered nearby, it would likely be the target of our first interstellar mission.
Brown dwarfs are smaller than the lowest-mass stars but can be dozens of times more massive than Jupiter. They are too low mass to sustain hydrogen fusion and so technically they are not stars by definition. (The term brown dwarf is also misleading because brown is not a color in the visible spectrum. Something like ultra-red dwarfs would be a more appropriate name.)
Among the biggest questions is why brown dwarfs aren't commonly found orbiting normal stars. However, some tend to hang out together in binary pairs of dwarfs. That said, they are found in the vicinity of normal stars but not gravitationally bound to them, as seen in the above photo of the Pleiades star cluster.