Did you know some scientists suspect our solar system may be a binary system and we might have a red dwarf out there somewhere in the far reaches of our solar system.

NASA says that most stars it has observed are in fact binary systems with two suns orbiting each other.
Red dwarfs are the most common form of star in our galaxy and it is quite possible there could be an undiscovered red dwarf star in our own solar system, on a vastly eliptical orbit of our sun.
Every 26 million years our Earth seems to suffer mass extinction events where nearly all life is wiped out. This could be explained by a dark star or red dwarf coming in close to our sun every 26 million years, triggering comet strikes by perturbing their normal orbits in the Ort cloud, and sending them into the inner solar system. This dark star could be very hard to see which is why it has never been found. Red dwarfs are very small and dim objects. Which explains why it has never been seen. It could be the closest star to our sun and we don't even know it.
Only around 10% of binary stars are of equal size. The rest are of unequal mass. This means the smaller mass star is on a much larger orbit and can take millions of years to complete a single orbit. The search is on for the dark star called Nemesis.