Space is unimaginably larger than humanity can conceive. There are objects in the cosmos that could very much challenge our currently held views on arts, politics, religion, and possibly what it means to be human. Still, despite the grandiose size, space is extremely empty. Sure, there are multitudes of particles that defy the idea of a vacuum, but these are extremely tiny themselves. So what is the chance that in all of this near void that there is some unknown hazard to the Earth lurking beyond our telescopes? Likely near 100%. On the other hand, the chance that this hazard is the hypothesized Nemesis star is well on the other side of the probability spectrum.
Within the geological record there seems to be a pattern of extinctions as every few hundred million years or so some drastic event occurs and life on Earth finds itself in a bind. We know that around 65 million years ago, we were bombarded by an asteroid and around 150 million years ago, another massive extinction event rocked the planet. The periodic nature of some of these events has lead to multiple hypothesises about the nature of our solar system and its interaction with surrounding neighbors.
Nemesis, if it is to exist, is a hypothetical brown dwarf that is significantly smaller than our own sun that orbits fairly far out of the system, possibly on a much more elliptical orbit than any object besides Sedna. As it orbits the Sun, Nemesis periodically passes through fields of comets and asteroids, sending them out of normal orbit and into the inner solar system. As to why it has not been detected, brown dwarfs are cool enough that they do not give off light in the visible spectrum and instead emit deep within the infrared range. Since there are very few infrared measurement devices in space, there is the possibility that such an object has escaped our observation.
Where the Nemesis hypothesis falls flat is that there are other available reasons for a periodic nature of collisions. The stars themselves are hardly fixed in the sky. As our local neighborhood circles the Milky Way, certain stars grow closer to each other then move away, only to come back on the next pass. Think of it like a merry-go-round where the horses move on slightly different paths while keeping an overall elliptical orbit. There is also the idea that not only does the sun go in an orbit around the galactic center but also moves up and down in reference to the galactic plane, which may subject the solar system to different settings space-wise that could lead to impacts and other catastrophes.
All-in-all, it may be interesting to have a cosmic partner in our sun’s dance around the galaxy, but it is highly unlikely. You never know though, space is full of surprises.
May the stars shine bright in your skies.