Suppose a civilization somewhere in the cosmos is approaching Kardashev type III status. In other words, it is already capable of using all the power resources of its star (4*10^26 W for a star like the Sun) and is on the way to exploiting the power of its galaxy (4*10^37 W). Imagine it expanding out of its galactic niche, turning stars in its stellar neighborhood into a series of Dyson spheres. If we were to observe such activity in a distant galaxy, we would presumably detect a growing void in visible light from the area of the galaxy where this activity was happening, and an upturn in the infrared. Call it a ‘Fermi bubble.’
Searching for signatures of cosmic-scale archaeological artifacts such as Dyson spheres or Kardashev civilizations is an interesting alternative to conventional SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the original civilization. This type of search is called interstellar archaeology or sometimes cosmic archaeology. The detection of intelligence elsewhere in the Universe with interstellar archaeology or SETI would have broad implications for science. For example, the constraints of the anthropic principle would have to be loosened if a different type of intelligence was discovered elsewhere. A variety of interstellar archaeology signatures are discussed including non-natural planetary atmospheric constituents, stellar doping with isotopes of nuclear wastes, Dyson spheres, as well as signatures of stellar and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is introduced in the discussion of galactic signatures. These potential interstellar archaeological signatures are classified using the Kardashev scale. A modified Drake equation is used to evaluate the relative challenges of finding various sources. With few exceptions interstellar archaeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.
Dyson notes that “a type III (Kardashev civilization) in our own galaxy would change the appearance of the sky so drastically that it could hardly have escaped our attention,” while Annis observes “It is quite clear that the Galaxy itself has not transformed into a type III civilization based on starlight, nor have M31 or M33, our two large neighbors.” These statements are reasonable but what would happen for a civilization on its way to becoming a type III civilization, a type II.5 civilization so to say? If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi  that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.
To get a sense of the time scale for a typical Fermi voyager recall that the escape velocity from the solar system from the vicinity of Earth is 42 km/s or ~0.0001 of the speed of light. It is notable that Voyager 1 is now traveling at a speed such that it will escape the gravitational potential of the sun. For an expenditure of 100 times the energy, the velocity could have been raised to 400 km/s. The diameter of the Milky Way is 60,000 light years so that a traveler going at a velocity of 400 km/s or 10-3 c would take 60 million light years to cross it. The sun takes roughly 200 million years to circle the center of the galaxy.
Newman and Sagan modeled the possible diffusion of interstellar civilization across space. One observation is that the time to colonize individual stars is not large compared to the time to travel between stars. The expanding front of the civilization might move by a diffusive or random walk process. However it might also go directly to the next star so that the front would move forward at a rate comparable to the space travel velocity. In that spirit an intergalactic civilization could envelop a galaxy on a time scale comparable to or even somewhat shorter than the rotation period of the galaxy.
Searching for ETI-driven artificial bubbles in a spiral galaxy is challenging because the structure pattern itself is based on voids of a sort.