Are we the only sentient species in the Universe? Not in the personal universe inside our heads, where we are always alone, but the physical universe of which our planet is an infinitesimal piece.
Let us dispense with the ugly neologistic oxymoron 'Multiverse' before it raises its misshapen head. The term 'Universe' by definition includes all of creation. It cannot be redefined as a part of something simply to satisfy some desire on the part of Post-Modern cranks to bolster their delusions.
There are at least three points of view to consider here, creationist, scientific and true believer.
True believers seize upon anything that remotely resembles evidence, in a dark cellar, at midnight.
Their history of breathtaking credulity is unrivalled since the dark ages, with obviously faked photographs and eyewitness testimony (the most unreliable evidence there is) being held up as irrefutable evidence.
There has been a publicly available report emanating from a "Sergeant Clifford Stone" who was allegedly involved in "top-secret government projects." One is tempted to comment that the nature of government is to classify everything, including the President's hat size: this man could be a haberdasher.
Be that as it may, this man, who was involved with the "Disclosure Project" conference in 2001, asserted with a straight face that "57 alien species have been catalogued!"
One is amazed at the prescience of H.J. Heinz, who foresaw this as long ago as 1892 in devising the slogan of "57 Varieties" despite the fact that his company's product list had over 60 items. Clearly old HJ had insider information, and probably contacts amongst the aliens, which accounts for those "holiday recipes".
Disclosure Project is an organization that claims to have access to 400 witnesses in government, military, and security organizations that will discuss government coverups of alien activity.
One is tempted to ask the obvious question; If the secrets of the Government are leaked with such impunity, how reliable are those entrusted to keep those secrets? The information provided by Sergeant Clifford Stone sounds remarkably like government disinformation, used in the same way as a new toy is used to distract fractious infants when lunch is late.
Creationists believe that the supreme deity made the heavens and the earth, all the denizens therein, but only one sapient species. This is alleged to have happened around four or five thousand years ago, in the course of a week, with humans being made on a Friday afternoon about teatime.
So much for that.
The scientific POV is that statistically the probability of sentient life somewhere out there is pretty high. There are, after all, somewhere in the region of 10 to the 100th power stars, many of which will have planets. To assert that none of these will be capable of supporting life is at the least, rather a poor bet.
To assert that some of these lifeforms are similar to our own is quite another matter: a significant portion of our atmosphere is comprised of a gas that is corrosive and toxic to many lifeforms; Oxygen. Had our remote ancestors not formed a symbiotic relationship with mitochondria, we would be less than a footnote to evolution ourselves.
Now we need another planet capable of sustaining life, where sentient life has evolved, and has by one means or another managed to survive in an oxygen atmosphere. Not impossible, but significantly less possible.
Finally, we must postulate that this species is far in advance of our own technologically, capable of space flight and all sorts of other things. The probabilities are, by this time becoming very small.
So, in summary, are we alone? Likely not, though the life out there may be so radically different from our own that we could pass it by and not recognize it for what it was. In fact it is questionable if we would recognize it for life at all. Our perceptions are, after all, limited by being oxygen-dependant mammals, living on a planet 93 million miles from a small yellow star.