In Stephen Baxter's sci-fi books about extremely far future technologies he envisions a race using what are called "cosmic strings" to manipulate space and time for their own benefit. Cosmic strings are hypothesized to be the leftovers of the creation of the universe; fragments of the Big Bang itself. They are actually the extremely high gravitationally bound material that had filled in the "cracks" that were created from the Big Bang and its subsequent expansion. They are hypothesized to be many light years long and perhaps even strecth across the visible universe. The civilizations in Baxter's stories use these cosmic strings to fold space and time for travel and energy usage. Cosmic strings have never been observed in nature, but they are theoretically possible to exist and do not seem to break any laws of physics as we understand them. Their existence could possible be made known by their extreme gravitational effects, such as from "gravitational lensing" or through "gravity waves". In fact, the large-scale structuring of the univere's galaxies seem to point to the possibility of Cosmic String influences. Looked at from a universal scale, galaxies seem to collect along what seem to be filamentous strings as if from like spider webs leaving behind large bubbly "voids" almost completely empty of galaxies. It is not entirely known what has caused this, but Cosmic Strings have potentially been implicated.
I would like to speculate on some objects in the universe that I believe could potentially be interpreted as manufactured or engineered objects. Now, I will say without a moment's hesitation that, for the most part, these same objects also have natural interpretations, so my speculations are just that and serve only to spark interest and wonder. However, even though they do have their natural explanations, they are by no means complete ones, which further their mysteries.
Pulsars were discovered in 1968 and were, at first, labeled LGM's ("Little Green Men"). They are regularly pulsing objects that emit radio waves in very quick regular pulses, sometimes as close to 1000 pulses a second! They are usually between 10 and 20 km in diameter; about half the size of Phoenix, Arizona! Because of their oddity they were thought to be captured transmissions by spacefaring extraterrestrial vehicles plying the space between the stars. They are, in fact, the remnants of supernovae; giant stars that have exploded. They are what remains of the original star's collapsed core, called a neutron star that has continued spinning at extremely fast rates of speed and emit bipolar jets of radio waves. The radio waves are emitted only in bipolar directions (2 opposite streams) , because of very strong magnetic field that surrounds the neutron star that directs the radio waves in these directions.
It many cases, the earth happens to be lying in the same direction as these sweeping radio jets and can detect their presence.
Now, why do I say that pulsars can be considered objects of artificial manufacture? A highly developed extraterrestrial society could use these natural objects to their great advantage as beacons, and communication devices and perhaps as energy sources. If they could manipulate and transport pulsars around the galaxy their great energetic pulses could signal and transmit communications across the galaxy. Has there ever been anomalies associated with pulsars? Yes, there are a recently discovered variety of pulsars, called "magnetars" that actually almost shuts down and then can turn back on. Magnetars emit much, much more powerful bursts of energy, usually in the form of x-rays or gamma-rays and have varying strong magnetic fields that power them.
Quasars were discovered in the late 1950's. They have been found to be some of the farthest objects in the universe many billions of light years away, which means they are also the earliest objects that formed after the Big Bang. They are extremely powerful objects that continuously emit large amounts of energy. In fact, they are the most powerful continuously-emitting objects in the universe. Their size is about or less than the size of our solar system, but yet can be seen out to nearly the edge of the observable universe! They are thought to be the very energetic precursors of galaxy nuclei. Their energies seen to come from black holes.
Although it's very difficult to speculate on the extraterrestrial civilization nature of quasars, I can imagine a Type 3 civilization using, or creating(?), the vast amounts of energy that stream from these mysterious objects. There was some early speculation that quasars are "white holes", or the exit points for the infalling material that enters black holes. The material falls in the black hole, passes through a "wormhole" and exits through the white hole. Could Type 3 civilizations be creating or utilizing these sources?
Gamma Ray Bursters
In the late 1960's gamma ray bursts were discovered. These very short-lived bursts of energy release more energy than anything else in the known universe. They come from random spots around the sky and are detected about 3 times a week. Most come from extremely distant sources; billions of light years away, for the most part. Most astronomers believe they are the collimated beams of gamma rays that are emitted when a very large star collapses to form a black hole.
Could some of these energetic sources also be extraterrestrially-manufactured beams for communications by Type 2 or Type 3 civilizations? The technological level of these societies would be sufficient to produce sources this strong and could be detected across the universe.