The universe is an ever expanding area with countless galaxies which contain thousands, if not millions of stars which hold around them several planets each. Given the size of the universe and the time it takes to travel those great distances between galaxies, I hypothesize that it will never truly be known if there is other life unless it is discovered in our own backyard, our solar system.
It is already known that Mars once held water and an atmosphere. Dually, it is known that one of the moons of Jupiter is covered in ice. There could have been life on Mars at one point as there may very well be life on the moon of Jupiter.
"Life" is being used a lot in this article and debate overall. "Life" in this article means from the complex organisms similar to humans and whales to the more simple single celled bacteria. This being said, it is not only ignorant to think we are alone, but just plain stupid. Even if the life on Earth was a mathematical coincidence, with the number of planets orbiting around their respectful suns, it is more than likely that the proper mixture of ingredients for life has been or is mixing on another planet.
Now, if like is being defined as intelligent beings such as humans, it is a toss up. With life comes mutation. Life evolves to survive change. If one of these planets that holds or held simple single celled organisms has the chance to evolve, or did evolve as it would want to, then there would be life out there. But at any one time, it is unknown if they have died out as we will one day do.
Are we alone? We might or might not be. It depends on your particular definition of life. Due to the vast distances between galaxies and neighboring solar systems, we may never know.