The universe is so big that it feels almost ridiculous to think that we are alone. This is actually called the Fermi Paradox: if the universe is infinite, then no matter how tiny the probability of life, it must occur an infinite number of times too.
The question, though, is whether we can make contact with another civilization. And then comes the solution to this paradox: although the universe is infinite, we only experience a portion of it, called the ‘light cone’ in cosmology. It defines the area of space from which we may receive light. If another civilization has developed a bit too far from us, their signals have simply not reached us yet. And unless we are moving in their direction, it might never reach us either.
In summary, it is not a question of whether there is life out there, it is whether there is enough life out there for us to find it out. And on that, only time will tell.
It has been said that there are more stars in the universe than there are grains of sand on all the beaches on the Earth. If only a small number of those stars had planets on them that life could live on, that’s still a HUGE number of possible planets that could hold life! Because of this, I would be extremely surprised if there wasn’t other life in the universe with us.
There are some signals that could tell us if a planet had life on it, even if that life wasn’t intelligent enough yet to start sending television signals into space. Scientists studying methane levels on other planets because it is given off in some biological processes (think cows farting!). This is only useful if you have the assumption that the life on the planet we’re studying works in the same way as life on earth – it could work in a completely different way!
A definite way to know that other life was out there would be to receive a message! Unfortunately we’ve only been seriously listening out for any messages from aliens for at most about 100 years, which is a tiny tiny amount of time compared to the age of the universe. We’ve also been sending out our own signals for about 80 years, but since they can only travel as fast as the speed of light this only includes a small part of our own galaxy. Nothing from further away would have even noticed that we exist yet!