If you're following a science website, there's a pretty good chance you are aware of the Fermi Paradox, but for those who aren't, here's a quick recap.
Given the high probability that alien life exists out there in the universe (bearing in mind the vastness of space and that we keep finding planets within habitable zones), why has nobody got in touch yet? If there are so many other civilizations out there, possibly at far more advanced stages than we are because of how long the universe has dragged on for, surely at least one would send out messages or probes, or do what we are doing: Desperately searching for signs of life? And if not, should there not be signs of galactic colonization that we can detect, if any civilizations have made it to the point that they can spread throughout the universe?
We don't chat to ants
One solution is that alien civilizations are out there and detectable, but we're still far too primitive to even know what to look for. Civilizations could be millennia in advance of our own, with tech we haven't even imagined is possible yet, making it impossible for us to find them.
As for why they haven't got in touch with us, physicist Michio Kaku summarizes it nicely, speaking to the Daily Grail.
“Imagine walking down a country road, and meeting an anthill. Do we go down to the ants and say, ‘I bring you trinkets. I bring you beads. I give you nuclear energy and biotechnology. Take me to your leader?’"
In essence, if we don't attempt conversation with ants, why would aliens get in touch with us? Further to this, even if we were to be looking directly at them, it's possible we wouldn't recognize it as life, just as a head louse can't really comprehend your head.
We're in a zoo
Another solution that's predicated on the idea that we're still extremely primitive, compared to alien civilizations out there. In this version, aliens are fully aware of us but forbidden to get in contact and allow us to develop in isolation. Slightly better than ants, it could be that aliens think of us more of a highly intelligent e.g. dog with whom we if they put the work in and develop their society, might one day drop an email.
They didn't find dinosaurs to be friendly
"My great fear is that we've in fact been visited by intelligent aliens," Neil deGrasse Tyson once said. "But they chose not to make contact, on the conclusion that there's no sign of intelligent life on Earth."
As a species, we're only been around for a tiny fraction of a tiny fraction of the universe's existence. What's to say they didn't already check in on Earthlings and found them to be quite hostile dinosaurs.
It could be that we're in the unfashionable end of the galaxy, or a period in the universe, where life is uncommon and too far away to make contact. One team suggests that as advanced civilizations get to the point where they are consuming unfathomable amounts of power, they will have a huge problem dealing with their heat waste, and as such will head for the cooler regions of the galaxy.
"In particular, we suggest that the outer regions of the Galactic disk are the most likely locations for advanced SETI targets," the team wrote. "And that sophisticated intelligent communities will tend to migrate outward through the Galaxy as their capacities of information-processing increase, for both thermodynamical and astrochemical reasons."
We're in a simulation
A favorite of Elon Musk, this idea is that as technology progresses, there's a chance that is higher than zero that we will eventually want to run ancestor simulations of our own planet. As such, we can't discount the possibility (or, some say, probability) that we are ourselves living in one of those simulations. It could be one where aliens haven't been factored in - maybe they'll be added in a later update, just like on XBOX - or that the simulation is looking at some other question. With the power to generate billions of simulations,
They're in a simulation
Once you've got the tech, why would you live in the boring old universe and explore it, rather than simply hooking yourself up to the matrix and having simulated fun.