Whether humans are the only intelligent life to have evolved in the universe cannot be answered with certainty, but a probability can be assigned to the proposition. Accurately estimating the number of advanced civilizations in existence is done by assessing the number of opportunities for intelligent life to evolve and contrasting that with the improbability of various stages in the development of a civilization.
The Milky Way contains between 100 billion and 400 billion stars with many estimates settling on 300 billion, and exoplanet inventories indicate an average of one planet per star. Of the estimated 300 billion planets in the galaxy, about 20 percent, or 60 billion, are in their stars' habitable zones where liquid water can exist.
Simple life arose early in Earth's history, almost as soon as water formed, which suggests a high likelihood of the same occurring elsewhere. More complex multicellular life, however, took an additional three billion years to begin, which indicates a low probability that such an event can occur on a developing world. After large life forms evolved, another 500 million years passed before an intelligent species arose and built a civilization, suggesting an intermediate probability for that event.
The likelihood of intelligent extraterrestrial life depends on how often all the necessary factors come together. If the final odds are less than 1 in 60 billion, humans may be alone in the galaxy. Outside the Milky Way, the universe has another 100 billion galaxies where life might exist.