The universe being as enormous as it is, thinking that we are the only intelligent life in it would be the pinnacle of human arrogance. Even if we put the universe as a whole aside, we only need to look in our own galaxy, the Milky Way, with its star density of around 100 billion, to realize that there is a huge chance we have neighbors. There are all kinds of theories flying around that weve already been visited by aliens, or that the incredible jump in technology by the end of the 20th and beginning of the 21th century was due to NASA receiving technology from extra terrestrials. The scary thing here is that, if aliens have the technology to travel across light years to get to Earth, we will pose no threat to them whatsoever. Dr. Stephen Hawking has a firm belief that if we are to be visited, their intentions wont be peaceful. Most likely the aliens would be here to either mine the Earths resources or colonize our planet as their own. Theres also a high probability that the invaders wont be starting a full out war with us as seen in so many movies, but instead introduce deadly pathogens thatll wipe us out without them lifting a finger.