First let me state my stance on this subject I think that we are not alone in the galaxy let alone our universe. This is because it is mathematically improbable that we are alone within our galaxy. Now I'm not saying I believe that aliens have ever visited earth or have advanced technology compared to us but I do belive that there is another intelligent civilization in our universe. This is something I cannot prove nor can anyone else but no one could disprove it as well (at least yet). Mabey someday in the future we will be able to prove or disprove alien civilizations.
Let me explain that I think of this as more of a intelligent discussion rather than a debate. There are about one hundred billion galaxies each with about one hundred billion stars. Also the planets out-weigh the stars 4 to 1 so even within our milky way galaxy there is about 400 billion planets. If we take just 1 percent of those and say that is how many planets are in the habitable zone we get 4 billion planets in the habitable zone. If we take just half of a percent and say that is how many planets have the necessary conditions for life we get 20 million planets that are able to sustain life. Now let's assume that some of these planets populations were destroyed and let's again say that only half of a percent of the planets that were unable to sustain life actually survived with out total extinction that makes the number of planets with life still on it today 100 thousand. This would mean there is 100 thousand other planets with life just within our own galaxy. (These percents are just estimates very low estimates if I might add). This is why this is improbable that we are alone even within our own galaxy let alone the entire universe.