Well, I did not mean not expansion.

Galaxy could be all colonised, but these colonies sophistically hidden and creating in not obvious ways that only supercivilization can come up with and create.

For example, turning sentients into an intelligent picodust that takes power from radiation and that appears to anyone else just as normal interstellar dust do, but it hides something else... + nano to pico or even planck sized Von Neumann style machines that convert entire planet interiors into computing substrate and repair it.

My point is that it is entirely possible that there are megacivs. with "megastructures", just not these too obvious like classical Dyson shells, rings etc., but much more advanced and clever designs that provide secrecy as a side effect (a desired one).

For example, would you consider planets that are appearing normal on the surface, but with interior of computronium or some other strange ultratechnology thing designed to mimic normal planet interiors (at least from the outside) as detectable by any technology?!

Even if they colonised many planets and covered half of them with solar collectors they would be still invisible for us.
Even if they use radio they would be probably careful enough to jam it to not go anywhere where it is not supposed to go (even w/o it we simply cannot detect unidirectional radio signals of communications power because they are ruined in noise around at the distance of Pluto).

Well,but you all seems to forgot that billion year old civs. would probably have intellect at least 12 orders of magnitude better than us and understanding them would be probably like expecting bacteria to learn the principles of nuclear fission.
Billion year civ. = existed 100000x longer than our civilization (if you take the dawn of our civilization as cca 8000 B.C).
I strongly suspect that civ. that's been around for so long can manipulate strings to create FTL speeds, store their information in cosmic dust and gas, master godtech plancktech, create virtual universes in planet interiors filled with immeasurably dense computing substrate etc.

With 400 billion stars in our galaxy, I suspect that at least a few of these "masters of the universe" quietly lurk our galaxy.

BTW I've read in one of our popular science magazines roughly before a year or so, that research on "intelligent dust" being able to function as little cameras, tracking devices etc. is being taken, so I guess that it would be extremely trivial for billion year old civilizations to basically make everything smaller than atom, only not the computing substrate clusters.
Also I think that blocking all the EM, electron etc. radiation would be as easy for them as pickin up a coin for us.

You have enormous energy radiating structures in your scifi OA universe, but that's 20000 years, not a billion year old civilization.
I think reasonable civilization would be cautious and never build things so obviously, instead developing the best ways to do it.

We can speculate endlessly, everything is possible.
What if we are in a simulation, bubble universe etc...

But one thing that angers me, is when someone says "we are alone because we don't see funky Star Destroyers with huge jets everyday in the sky" or "hope there is no one else because I am paranoid and I cannot never understand that a defeat is always possible and let's kill everyone alien just for safety" or "Aliens do not exist because if they existed they would have colonised the Earth even if it would be totally unfit to them and they would have no interest of watching its biosphere from distance and study its uniqueness because they don't have 400 billion systems just in this galaxy systems to colonise and 10000000000000000000000 stars to explore, oh, and ofcorse they would leave bones in the earth as dinosaurs did because they sure have bones and bury their bodies in the Earth because even trough capable of travelling 100000 ligh years they cannot extent their lifespans".

And what if there are even stranger forms of life, intelligence and civilisation, such as neutron star dwelling creatures in a rich wonderful biosphere using nuclear and electrical reactions in place of chemical, with time being that microsecond is like a minute for them, with size of molecules, living on a body with 1 mm thick neutrino atmosphere where time goes slower on the poles than on the equator, with hundred billion G gravity being normal for them, and their biology gradually evolving and their metabolism slowing dowing as the neutron star cools?
These creatures would not even recognise Earth at a first glance, rock being hard vacuum for them.