We are only just learning of the mighty Galactic Civilizations which lie out in the depths of the Milky Way.  We have even met one, the Tsubar'ey.  But what can we infer about those civilizations which we may never meet?

The interstellar phase of a society generally lasts only a couple of thousand years or so in which the civilization expands and explores in phases over a region several hundred to about two thousand light years across, depending on the species.  Those that are galactic civilizations have reached a point where interstellar travel and expansion are relatively cheep and regular, usually through the use of beamrider and/or wormhole technology.  Using beamrider technology and self replicating nanotechnology they can continuously expand their sphere of knowledge at a constant fraction of the speed of light.  They will generally have explored the entire galaxy in 72 to 800 thousand years, which could be considered an intermediate period of development either as a late interstellar level or early galactic level.

Most galactic civilizations are aware of each other and have probes and possibly unmanned outposts within each other's territory.  Using their probes or diplomatic missions all galactic level civilizations are probably mutually communicative with each other to some degree, a state which could be considered a mid-level or possibly "galactic-proper" development for galactic civilizations up to 180 thousand years after the period of exploration.

These galactic civilizations probably have achieved a "presence" throughout the majority of the galaxy or at least the galactic habitable zone, though for the most part their presences outside the "polity proper" is entirely in the form of unmanned exploratory or diplomatic probes, though in the latter case some civilizations might find ways to send "manned" missions to the other galactic polities so more first hand and possibly culturally relevant information could be sent back to the "polity proper", this is probably a practice employed by the Tsubar'ey given what is known about their motives.

Most galactic civilizations would expand slower than before; being capable of selecting worlds ideally suited for their needs, rather than having to "settle" for those that might only be marginally habitable or already have a native intelligence taking up the usable resources.  They can also focus on expanding their infrastructure through the use or beamrider or wormhole networks and would be capable of impressive megascale engineering.

Another strategy likely employed be galactic level civilizations is some level of cultural assimilation of interstellar and lower level civilizations into their polity as opposed to direct colonization, and may explain the slower rate of expansion of their polity proper, as such diplomacy could take hundreds or even thousands of years.

Most galactic civilizations probably will only control territories about 15 thousand light years across at their height and may also be limited by natural barriers like the spiral arms they reside in, such as the ancient Eiyogsha and the current Far Sagittarian civilizations.  Most cannot sustain the resources or posses the drive or interest necessary to colonize the largely lifeless, hostile or barren rim and core regions of the galaxy.  The only current galactic civilization at this time making efforts to colonize outside the galactic habitable zone is the Magni civilization, who have expanded hundreds of light years into both the core and rim regions of the galaxy.  Given this, a "late" galactic civilization would either be one in the process of decline or one that is capable of exploring neighboring galaxies.  Those that can sustain such efforts generally come to encompass the entire galaxy during their height and may eventually come to control several neighboring galaxies as well and generally become successful intergalactic civilizations.

Galactic civilizations might attempt to escape a collapse by migrating to a new galaxy and re-starting their entire civilization; this may have happened with several civilizations in the past, as well as multiple times with the Tsubar'ey.  Some civilizations attempt other strategies but these are generally less successful and have been know to actually accelerate the collapse.  In the most sever case this results in a catastrophic upheaval that may leave a "barren" region in the galaxy essentially devoid of intelligence, as is the case with the current Southeastern Void encompassing almost one quarter of the galactic habitable zone.  Others do not so much as collapse but become so diffuse and spread out that they no longer maintain cohesion either through choice or lack or resources as occurred with the Eiyogsha and probably many others.

Over billions of years many civilizations have spread across our galaxy and in all likelihood most advanced civilizations in the galaxy are in some way or form descended from an earlier civilization either as a result of technological assistance, intentional provolution or having naturally evolved on a world terraformed by a previous civilization and later abandoned.  Some of these are able to trace their lineage back to one or another of the earliest civilizations in the galaxy billions of years ago.