We are not alone in the universe. It is too vast, it's immense size, multitude of planets similar to ours, and the recent discovery of key amino acids for DNA in a meteorite, create the probability that there are other technologically advanced species
Assuming Drake's equation, a complex formula that supposedly predicts the number of advanced technological species at any given time, is correct and mathematically accurate, we can be given estimates ranging from 5000 species to we are probably alone by changing the number of years predicted that a technologically advanced species will survive before an apocalypse, complete genocide, their own species burnout due to overuse of reesources, etc. Let's be generous and say there are 50 technologically advanced species in the galaxy all capable of space travel and broadcasting radio waves or other communicative waves through space. The probability of us seeing this race is highly improbable. We are still in the early stages of space travel; we have only sent two people to the moon around 50 years ago; the event hasn't been repeated, and there has been no manned mission to any other planet to date. With our present technological abilities to travel space, it would probably take us around 50 to 100 years to start manned missions to Mars and other nearby planets and another few hundred at least to start trying to reach out of our home solar system. With this in mind, that we cannot search for aliens ourselves except for what contact they choose to give us, it remains improbable that we would discover other species.
With how technologically advanced we are, lets say we are in the 50% range, placing us as a median for all other present intelligent species. As we just became capable of space travel and broadcasting television and radio, that leaves 25 species that have the potential of contact. Along with this the percent is random, but a good place to start, on the other hand if someone was pessimistic and determined we are the most technologically advanced species, no contact would be available due to the immense distances placed between civilizations. Also if someone said we where the least technologically advanced, we could assume all other species have the ability of space travel etc.
Another important reason for limited or no contact with our species is the fact that we still have not united under one banner of humanity and we are still in the long period of brief periods of peace dotted by short civil wars between our species. If the other species have in fact joined in an intergalactic community, they would, in their infinite wisdom, watch us until we have achieved a lasting peace and united species that would insure our species survival and no chance of having to pick sides in human battles. Along with the fact that we are aggressive in nature and very destructive as a species, any intelligent species would probably find it easiest to leave us with our own problems to work out amongst ourselves before we are deemed able to deal with an intergalactic alliance of technologically advanced beings.
Although there is no contact with our species as of yet, Drake's equation along with several others can be used to give us a fairly accurate look into how many intelligent species there are roaming the stars. With our present destructive behavioral properties, it remains up to debate wether technologically advanced species are simply choosing to leave us alone, or whether they haven't appeared because they don't exist.