"A spacefaring civilisation, even one relying on craft travelling at far below the speed of light, would be able to colonise the entire galaxy in a few hundred million years. It therefore follows that if intelligent, technologically capable life forms had emerged elsewhere in the Milky Way, they would probably have done so long enough ago that they would, by now, be everywhere — which evidently they are not. This line of reasoning suggests humans really are the only intelligent life in this particular galaxy.Dr Griffith has checked out a large number of galaxies, but there are at least one hundred billion in the observable universe so he's sampling at a ratio of one in a million. If there are aliens with super-powers anywhere in the universe, this approach - scaled up - seems an effective way of getting the evidence in.
"Perhaps, therefore, the search for aliens is looking in the wrong place. The calculation that intelligent life will rapidly colonise its entire home galaxy — first made by Michael Hart, an American astrophysicist, in 1975 — suggests it is not other solar systems which should be scoured for little green men, but other galaxies. And this is just what Roger Griffith, an astronomer at Pennsylvania State University, has done."
"Dr Griffith reasoned that a galaxy inhabited by Dyson-sphere-constructing aliens would have an unusual, infrared-rich and visible-light-poor spectrum. With the aid of an American space telescope called the Wide-field Infrared Survey Explorer, he searched 100,000 galaxies for such spectra. What he found, as he reports in the Astrophysical Journal, is tantalising.
"No galaxies appeared to host civilisations that were using more than 85% of the available starlight as a power source. Fifty, however, were red enough to be hosting aliens gobbling up half or more of their starlight. Since even the most enthusiastic colonists would not, presumably, set up shop around every single star, and also because realistic versions of Dyson spheres would not totally enclose a star, these galaxies might indeed be the empires of individual alien species.
"Power-hungry aliens are not, sadly, the only explanation for the spectra Dr Griffith has found. More prosaic things, such as vast clouds of interstellar dust, might produce a similar signal. Nevertheless, these 50 unusual galaxies (and also 95 more which had spectra that were weird in other ways) might repay further study. The odds are that Dr Griffith’s discovery will have a humdrum explanation. But it is just possible he has answered the age-old question of whether humanity is alone."
Look at this from the viewpoint of physics, not biology - or 'life-sentimentalism'. What we call life (or more properly, biological ecology) is an exponential process of environmental self-reconstruction. Inorganic-stuff turning itself into ever more life-stuff. A chemical self-organising diffusion process powered by free energy.
Ignore all the fine-grained details of 'intelligence', 'technology' and endogenous 'lofty aspirations'. From a sufficient number of millions of light years away, the galactic spread of free-energy-exploiting alien life is indistinguishable from the runaway infestation/transformation of a sterile, pristine galaxy by 'grey goo'.
There are people who rather like galaxies as they are now. Should we call them misozoic (after this and this)? You read it here first.