Are We Alone in the Universe?
This question can be split into two parts...
1. Is the Earth the only planet in the entire universe that harbors life?
2. If there is life elsewhere in the universe, are humans the only intelligent beings?
There is no physical evidence confirming that life exists anywhere in the universe except on Earth. We haven't found any artifacts on the moon or on any of the planets we have sent probes to. We have found no fossils or remnants of life on Mars. We haven't detected radio waves or any other attempts at communication. The closest we have come is an unusual signal known as the "WOW" signal, that could not be found again after it was discovered.
Of course there are people claiming to have removed "technology" in the form of alien implants from people's bodies. None of these supposed implants have ever been scientifically verified. The most likely origin for these implants is nothing more unusual than foreign bodies (shards of glass, splinters) that have entered the subjects body and have been subjected to the bodies natural protection processes of surrounding the offending object with tissue.
Having no verified physical evidence though does not suggest that life does not exist elsewhere. We have only just begun to explore the universe, and we are severely limited in our methods of exploration. If you searched a square meter of the Sahara Desert for water and found none, you couldn't infer that there is no water anywhere on the Earth.
The best circumstantial evidence for the existence of life elsewhere in the universe includes the sheer size of the universe and the fact that life exists at all. If life can evolve on Earth, life can evolve on other planets. It would actually be very surprising if life hasn't evolved on thousands of planets, or given the size of the universe hundreds of thousands or even millions. Scientists estimate that at least10 billion habitable Earth-sized planets exist in the Milky Way galaxy alone. See this interesting information from NASA. According to the best estimates there are around 100 billion galaxies in the observable universe. This leaves a lot of room for life to evolve and a lot of unexplored real estate in our search for such life.
Supporting for the above includes the diversity of life on Earth, the many different environments that harbour life and the very real possibility that life on Earth has begun independently more than once. The jury is still out on this, but there are life forms living on Earth in the most extreme environments, such as off the nutrients supplied in the deep ocean by hydro thermal vents, never seeing sunlight. Is it more likely that this type of life evolved independently or that the creatures there somehow adapted to the environment?
Life itself seems to be very tenacious. We have examples of life in the air, underground, on land and in the sea, and even as stated above, surviving off the nutrients supplied by thermal vents in the deep ocean. We have examples of extremely hardy lifeforms that can withstand radiation at levels that would kill a human in next to no time. For an example of extreme life you may like to take a look at the tardigrade or water bear.
As to the non physical evidence for intelligent life existing elsewhere in the universe, you just have to look at the varying levels to which life on Earth has evolved. Again we only have a sample set of one, but in this sample set evolution seems to favour anything that will give an organism an advantage. This includes enhancing physical capabilities just as much as enhancing intelligence. It's fairly safe to assume that if life has evolved on other planets, that life would endure the trials of survival just as life does here on Earth, and those trials of survival will bring about evolution, which in turn will eventually lead to intelligence.
Given the extreme diversity of life on Earth, the sheer size of the universe, how little we have explored of the universe, and the extreme types of life we have found here on Earth, the possibility of life existing elsewhere on the universe is a very credible belief. The possibility of intelligent life existing elsewhere just as credible.