OK, let's put all this into perspective. To start with, I have always believed there has been, is now and will be in the future, other intelligent civilisations in the universe. I used to be unshaken in the belief that our galaxy had many intelligent civilisations right now, a kind of 'Star Trek' universe where intelligent life is out there, a mere few light years away. I still hope that is the case.
But, if one starts to look objectively at all information and evidence, I am now coming to the conclusion that we are probably the only intelligent civilisation in the galaxy right now. That does not mean I do not think there is microbial life out there. I actually think the galaxy is teeming with that. I also think plant life and perhaps animal life exists in the galaxy but to a far lesser extent.
If we cast our minds back to my first post on this subject, I mentioned Fermi's Paradox. The question that if the galaxy is teeming with intelligent civilisations, where is it? Even in a galaxy where we may find it difficult to comprehend its size, intelligent life is either 'teeming' as Fermi stated or it is not ie it is less frequent.
If, it is accepted the galaxy is not 'teeming' with life then, defacto, we are on the road to accepting the paradox as being correct and do not really need to consider it further.
However, if we hold onto part of Fermi's Paradox that the galaxy is teeming with intelligent life, we must ask as did Fermi, where are they?
In essence, the paradox could be dismissed if we saw evidence of this teeming intelligence throughout the galaxy. We should be able to easily detect their communication signals such as radio or light beams. It is suggested that aliens may be using some other medium to contact each other that we do not yet have access to. But, is it reasonable to assume that every other intelligent civilisation that exists has discovered this 'other' medium? Is it reasonable to assume no other intelligent civilisations would still be using radio or light \ lasers for communication?
Besides this, the most common element in the universe is hydrogen. Any intelligent civilistion would recognise this. Should any intelligent civilistion want to attract or listen out for other developing civilisations in the galaxy, one radio frequency they would surely listen out on is 1420mhz, the frequency of hydrogen. SETI has listened to this frequency for over 40 years. No verified credible lighthouse signal from another civilisation has been detected at this frequency. WOW! and signal SHGb02+14a are interesting but both enigmatic. Both are discounted by SETI. SETI also monitors millions of other radio frequencies, there is also optical SETI albeit far newer and still no verifiable candidate has been proven.
It stretches credulity that all this teeming intelligence is using some other communications medium while totally ignoring the technologies a developing intelligence we ourselves would start out with.
Searching for means of communication isnt the only method whereby an intelligence would show themselves. In a galaxy 10+ billion years old, we are newcommers on the block. There has been plenty of time for all this teeming intelligence in the galaxy to reach out and travel to the stars. Even at sub light speed, a single civilisation could colonise the galaxy from one end to the other in a few million years. Estimates put this at between 5 million and 50 million years, a blink of an eye compared to the age of the galaxy. Yet again, the paradox comes into play when we ask where is the evidence of a colonising civilisation? They either colonised the galaxy and therefore should have left behind evidence of this or they did not.
The lack of evidence must mean either they did not colonise the galaxy or, there has been no one capable of colonising the galaxy.
So we are led to believe in a galaxy teeming with intelligent life, none have even tried to colonise it. Surely then, we should see evidence that at least some of this teeming intelligence would explore some of the galaxy either in person or by other means?
As surely as we have cast the Pioneer and Voyager probes out to deep space, others more intelligent would do something similar. von Neumann or Bracewell probes would have been sent out to gather information and indirectly communicate with other civilisations they fell upon. Attempts have been made to activate any such probes in and around our solar system. This has been unsuccessful.
And what of a civilisations growing demands for energy? Dyson spheres or variants of them, should surely be common amongst more intelligent civilisations? We, at our level have explored the possibility of such constructs. Whether they be solid surface constructs or merely a ring of satellites, even Dyson himself suggestted the effect such a construct on a parent star would make it stand out and be a beacon of an intelligent civilisation. Yet we see none.
It is also proposed by some, that we couldnt possible communicate with alien civilisations because we'd be so different. But would we? Would every intelligent civilisation in this galaxy supposedly teeming with life all be totally so different that communication would not be possible? Its akin to saying that stone in your garden is alive but not in a sense that we recognise. In those circumstances communication may be impossible with that rock or that alien. But every other intelligence is the same?
Then there is the 'zoo hypothesis.' The idea that for some reason, the galactic federation has decreed that developing civilisation such as ourselves be left alone until they reach a certain development. This is fatally flawed however. For one, it depends on some form of federation to agree to and abide by the rules. If the galaxy is teeming with intelligent civilisations, they would have to be as one in agreeing to leave well alone. Even then, the chances are that one or more would break the 'prime directive' for some reason or other.
The zoo hypothesis doesnt work especially if all intelligence isnt benevolent. Its safe to assume that if the galaxy is teeming with intelligent life, some would undoubtedly be peaceful but equally, some would not. They would not hold with keeping to the hypothesis.
Perhaps finally, there is the rare earth hypothesis. Essentially, this hypothesis argues that a series of fortuitous circumstances must occur for multicellular life and therefore intelligence, to develop.
The Rare Earth Hypothesis dictates these circumstances to be the galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, the size of the planet, the advantage of a large satellite, conditions needed to assure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans etc.
In order for a small rocky planet to support complex life, the Rare Earth Hypothesis argues the values of several variables must fall within narrow ranges. Although the galaxy is vast and could contain many Earth-like planets, the fortuitous circumstances that potentially led to intelligent life here may not be repeated elsewhere.
In short, no one single dataset can point to the fact that we are probably alone as the only intelligent civilisation in the galaxy right now. But, when all are taken and considered together, the likelihood is that we are, alone.