This is a very tricky question, as we have no proof either way, so it would not be scientific to cling closely to either “yes” or “no”. In the absence of any real means of testing this question experimentally, it comes down to belief.
That said, I do believe that there is other life in the universe. In fact, I do believe that there is other intelligent life in the universe. I base this on the fact that the universe is really really big, and it seems unlikely that this one planet would be the only place where life evolved. Seems too improbable that we are alone. Can I prove this? No, but it is my belief.
There is an formula that is used to estimate the number of intelligent civilisations. It is called the “Drake equation” after Frank Drake, who came up with it in 1961. Basically, it uses estimates of the star formation rate, the fraction of stars that have planets, the fraction of planets that could sustain life, and so on. It throws all that together to give an estimate of the number of sentient aliens. But we have to take this with a large grain of salt — there are wide uncertainties on the numbers that we put into the equation, so the number we get out will also be very uncertain!
There is also ongoing work done to try to answer the question once and for all, by actually finding intelligent life. Probably the most famous centre for this work is the SETI project, which does what it says on the tin: SETI is the Search for Extra-Terrestrial Intelligence. Nothing found yet, but the search continues! Of course, the best way to answer your question would be to actually find something — or someone — else out there!
I don’t think aliens exist. If they did – where are they? We’ve not seen or heard any signals that could be from intelligent life. We’ve looked pretty hard for a long time at the sky and never seen anything. This means that life on other planets is either extremely rare or intelligent life doesn’t exist. That is why we need to take particular care of this planet. To quote Carl Sagan: “It’s the only home we’ve ever know.”