You also need to take into account the life-span of an intelligent species. This reflects on whether that civilisation is likely to be discovered or not. Our galaxy is approximately 100,000 light years across. That means that for a radio signal (travels at speed of light) will take 100,000 years to travel across our galaxy. As a species, we have only been making radio signals for the last 80 years? Therefore, if there was any other 'civilisations' out there, unless they are within 80 lyrs, they won't even know we are here!
Also, going bac to the drake equation, from our own experience, intelligent species do not come along very often. There has been life on earth for the last 4 billion years. It has taken this amount of time for the first intelligent (technologically advanced) species to evolve! There could have been many species come and gone in that time!
The conclusion you could draw from that is that, intelligent species may come and go and never be fully aware of each others existence! There could be a technologically advanced species on the far side of our galaxy! We won't know for at least another 99,920 years or so!
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