Let's put it in the simplest terms.
There are are a very large number of planets in the universe, all of them with varying physical attributes, but all composed of an assortment of elements and compounds whose properties are known to us.
The conditions necessary for life as we know it to develop are not that great in number, and it follows that there may even be other life forms that need less.
Given the above, it is highly probable that some life form exists, has existed, or will exist somewhere else in the universe.