I like how KS Robinson handled the question in the Mars Trilogy.
This is one of the long-simmering questions of the trilogy of novels, so don't read further if you plan to read the books (Red Mars, Green Mars, Blue Mars).
In Mars Trilogy, life at the sub-cellular level on Mars and life in the universe at large is seen as almost certainly to exist, but so distant either on large or microscopic scales that humans will basically evolve so far that they can control space/time before we could find it.
Seriously, it's explained very well. I do not do KSR justice, but briefly it's like this...
Potential proto-life forms and such things get smaller and smaller...these are things that are classified and tested as methods of life evolving from inert material, and they find some bits of quasi-organic compounds in tectonically active sub-surface ice, but they can never completely prove that the substances viability to evolve into life is either due to contamination or not.
On the galaxy and universe scale, KSR takes a modified "So where are they" stance to *intelligent* life. It's like a possibility horizon that extends outward all the time from earth in all directions.
We would have heard from anyone close enough to visit in any sensible time and relay that info back to earth. Plus life-extention and quasi-immortality via mind-uploading are in thousand year timescales compared to 9^10 year time-scales for sending probes or visiting even with faster-than-light travel (which isn't yet invented in the books) considering the galaxy and universe is so incredibly vast.
Basically the answer is "We would have met them by now, but we'll never know for sure, given all we can imagine 'knowing' might mean in any context, limited by space-time."
The books have a small aside about a group that makes a quasi-generational ship to begin a trip to a new star, which will 'only' take 45 years, in a time when humans live to be 300 years and older due to technology that helps put it all to scale.