English: Composite image of the Earth at night...
Our planet has been around for (approx) 4.5 billion years. In that span of time, Earth has gone from a complete lack of life to the society that exists today, and (presumably) in the near future we'll be expanding outwards from earth. Life grows and expands at an exponential rate.

It's estimated that the sun is a third-generation star. Meaning, that two generations of stars have been created, burned through their nuclear fuel, and died out. The lifetime of our star is only slightly longer than that of earth - a few tens of millions of years.

If it only took life 4.5 billion years from formation to space travel, and the universe has been around for two ~5-billion-year generations (in reality, it's estimated at 13.7 billion years), then life that began forming near the start of the universe, or within the next few billion years, would likely have attained space travel by now.

The universe is really, really big. Bigger than most people can even wrap their heads around. Big enough that we can assume that life exists somewhere else. So, given that (A) Live exists somewhere in the galaxy, and (B) It has attained the ability to travel through space by now, and (C) Life expands incredibly quickly then, by all accounts, we should have found overwhelming evidence of its existence by now. Our galaxy should be absolutely teeming with life, just as it appears to in so many science-fiction movies. so where is it ?