Parallel Universe
According to an international team of scientists, the aliens could be living in a parallel universe. The researchers from England, Australia, and the Netherlands concluded after running the Evolution and Assembly of galaxies and their environments.
In their research, dark energy, which is opposite to gravity, became a crucial factor. They found that even if dark energy is a hundred times the amount in our universe, galaxies can be formed, forming a parallel universe and becoming home to Aliens.
Drake's Equation
According to Drake's Equation, there Could be 3500 to 4500 active alien Civilizations in our galaxy.
Hibernation theory
According to this theory, aliens are incredibly advanced and spend their lives hibernating, waiting for the universe to cool down.
Trapped in Deep Oceans
Alien life forms might be trapped in deep Oceans, where they're trying to overcome a significant hurdle to reach the surface.
Lack of Examples
The only example of life is lived on earth, and there can potentially be other life forms beyond our imaginations.
Well, we've known for a long time approximately how many stars exist. We didn't know how many of those stars had planets that could potentially harbor life.
"Of course, we have no idea how likely it is that an intelligent technological species will evolve on a planet," says Adam Frank, professor of physics and astronomy at the University of Rochester. But using our method, we can tell exactly how low that probability would have to be for us to be the only civilization the universe has produced. We call that the pessimism line, then a technological species and culture has likely happened before.
Using the approach, Frank calculates how unlikely advanced life must be if there has never been another example among the universe's ten billion trillion stars or even among our own milky way galaxy's hundred billion.
There are plenty of other ways to look for life that have been tried or suggested. Here are few ways
Looking for biosignatures
Over time, the biochemistry of billions of creatures can transform a world in distinctive ways. By analyzing the wavelengths of light that come through the planet's atmosphere, we can look for gases that suggest the presence of life.
Look for the techno signatures.
If we see signs of life on another planet, that won't tell us whether it is inhabited by mindless green slime or sentient city-builders. A more reliable approach to look for kindred spirits is to look for chemicals that only intelligent civilizations can produce.
Megastructures
Aliens might have built enormous engineering projects like Dyson spheres, named after the physicist and engineering free man Dyson, which are swarms of devices around a star that harvests energy and has nothing to do with vacuum cleaners.