Although there has been no known and verified contact with intelligent extraterrestrial life, some form of life may exist elsewhere in the universe, even if it isn't on a human level of development. Given the size of the universe, Earth is highly unlikely to be the only planet that supports life.
According to the Search for Extraterrestrial Intelligence Institute (SETI), one theory regarding life is that it develops wherever conditions are possible for it to develop. For example, microbes are hypothesized to exist deep within the underground oceans of Saturn's moon Europa, as well as to have once existed on Mars. The SETI Institute regularly searches for narrow-band electromagnetic emissions that human instrumentation can detect and denote an artificial signal indicating intelligent life at a human level of technology. As of 2014, the Institute only conducts passive experiments, searching for signals, and does not send signals that could be detected.
Astronomers are continually searching as well for Earth-like exoplanets that may support or contain life. Only a few are in the Goldilocks zone, the distance from the star at which liquid water can naturally exist. Even then, the atmospheric pressure has to be taken into account. Venus-like pressures, for instance, would crush all but the hardiest microbes.