To assert we are alone in the universe is to assume that all we know is all there is. There is no science on earth that goes by that assumption. Even cosmology itself, and nuclear physics is based on the certainty that there are worlds smaller than are own. There are also worlds, Jupiter is one, in fact, that are larger than our own.
Life on a world like Jupiter would have to be gaseous, and not at all like what bipedal homo sapiens is based on with our carbon bias toward life forms such as those around us. It would be truly bizarre to even find hominids of any kind. Think of how rare they are on our own world. Only one race of earth is truly bipedal, and no Captain Kirk, no matter how charming, could rouse more humanoid aliens into being by our own self referential bias.
Where the outrageous claim of being alone came from is interesting. Most of us, in our lifetime have never met anyone who thinks we are alone in the universe. Have you, for example ever known anyone who believes our earth is the only inhabited world?
Although most people believe in god, several in angels and far fewer in just life as we understand it on earth, most everyone believes in other places than earth where life exists. Nature plays an eternal game of many more seeds than beings, and many more habitats for niches for life than just carbon based, air breathing bipeds.
Life found its way to begin, thrive and spread on this planet. There is no reason to think that other planets, even in our own solar system could not have had their goldilocks moment in the sun as well. Mars is cold now, Venus is too hot. This does not mean they have always been so, or will always be.
That said, life is clearly more rare than space. Space is everything in between planetary bodies and there is a lot of space. There is so much space, that by the very distances of inter-galactic distance, there is little hope that anyone from even the closest star system could send and receive a message back in anything less than many years of waiting. There is so much time delay in the distance between most visible stars and our sun, that even if we were to get any message recieved or sent, it would be thousands of years of time between missives.
Then, again, there is speculation that perhaps a shortcut, often called a worm hole, connects the fabric in space like a button through a quilt, that may virtually be in both places at once. One way to travel across the fabric of this wide quilt is all the way around the breadth and width of it, but a shorter way is directly through to the other side. This would require a technology that is far beyond our ken and comprehension.
Never has anyone I've ever met believed we are alone in the Universe. Yet, the difficulty is understanding that although billions of earth like worlds do exist, we have not likely ever crossed paths. Perhaps, except in deep space time, which to our perception is about as close to eternity, (that is to say in the fulness of the time it takes a species to evolve, thrive and die away) as we will ever observe. As to all the reports of 57 kinds of aliens. These kinds of reports have always been with us, gods from the skies, angels, elves, leprachauns, fairies, demons and more, they all show up on earth sooner or later,
Yet, unlike true extraterrestrials, these visitors, especially those in UFO's and looking very humanoid in appearance, DO come from space, the space between very imaginative human's ears.