As humans, we tend to think that we have a reasonably good idea of what the universe is all about. We live on a small blue planet orbiting a star on the fringes of a thoroughly unremarkable galaxy in a universe that extends basically forever. No matter how far out you go, you’ll always feel at home, because the universe is pretty much the same in every possible location. Because of this, it’s suggested that the laws of physics don’t change and that what we find in our galactic neighborhood we’d also find billions of light years away. This homogeneous theory is called the Copernican principle, and it’s an axiom on which much of our scientific knowledge about the universe is built. It is also, very possibly, wrong.
In recent years, scientists began noticing something a little bit off about the structure of the universe. By analyzing the light from distant galaxies, they were able to tell the relative speed and direction in which these objects were moving. The strange thing is that, rather than flying apart like most things in the universe, some of these distant galactic clusters appear to be caught up in a sort of current, speeding at unimaginable velocities (about two million miles per hour) along a specific path. Scientists have coined this phenomenon “dark flow” because, honestly, they really don’t know what’s causing it.
For gravity to be acting on these clusters the way that it seems to be, there would have to be something massive waiting at the end of the path. By massive, we mean something potentially much bigger than anything that we’ve ever observed in the known universe; something big enough to absolutely dwarf the galactic clusters being sucked towards it like dust to a vacuum cleaner.
But if there is something out there so massive that it’s possibly “tilting” the universe, why can’t we just take a look and see what it is? The answer is that whatever is causing the pull lies outside the bounds of the observable universe, which has a radius of about 45.7 billion light years. That means that in the nearly 14 billion years that the universe has been around, light from beyond the known universe hasn’t had enough time to reach our telescopes.
So what could it be? One attributes it to some sort of exotic, as-yet-to-be conceptualized matter that affects physics in weird ways. Some have even suggested that a neighboring universe could be causing the pull.
Yet whatever it is, it does throw a wrench in the idea of universal uniformity. If there are unknown supermassive structures in other parts of the universe, why aren’t there any near us? Still, we should be thankful that there aren’t, really, because with a pull as strong as what has been detected, our orderly galactic neighborhood would probably get stretched out into celestial spaghetti and sucked into oblivion.