Is it possible that in our solar system or galaxy there is such a civilization so higly developed that we are "ants" unable to even be aware of their presence, in this very moment?
Yes. We know about hundreds of planets around other stars here in our galaxy; and we do not have the technology to capture an image of even one of them. Not even around our nearest star. If we can't capture an image of a planet a few light years away, we certainly could not see any structures on it. The Milky was is about 100,000 light years across; a civilization on the other side could have individual structures as big as our entire planet and we would not know it.
how can such a great civilization be "invisible" to us that we listen to radio signals coming from outer space and looking in the deep space with telescopes?
They could be too far for us to see. You aid "at this very moment", and we cannot see that far with a telescope. As for radio waves, they could be too weak; but we should imagine that most technological civilizations would quickly advance, just as we have, to the point their communications are no longer by radio, but by more accurate and efficient (meaning using very little energy) point-to-point laser, or cables (like us), be they metallic or fiber optic or some other material.
Perhaps most importantly; any leakage from their communications systems would be unintelligible to us, because it would look like noise. Data compression reduces patterns in signals, and the better it is, the fewer patterns it leaves. This is always a good idea, the speed of light is a universal speed limit, so data compression means fewer bits need to be sent and that means faster communications. If our technology continues, we will still be using data compression a million years from now, it is an "evergreen" benefit no matter how advanced we become. For a very advanced civilization any messaging leakage or missed targets would look like random background noise, swamped out by the real random background noise of a trillion stars churning out massive radio waves in the process of their very chaotic fusion process.
Is it possible that these civilizations will reach a degree of evolution such as their life form will be like energy-based, magnetic-based, non-material or purely spiritual?
Almost certainly not. This is hand-wavy fiction; all life as we know it is a biological machine, all electrical energy that does any kind of work does it by affecting a physical object made of atoms. Stars get their energy by fusing atoms. Our physics explain virtually every physical phenomenon known to man, and although I am in the camp of people that consider our physics incomplete and probably wrongly formulated (due to the serious incompatibility between general relativity and quantum mechanics), it is more like being in the bullseye of the target without hitting the precise center point of the bullseye; it has been exhaustively tested against reality without failing, and cannot be far off the mark.
It doesn't allow "pure energy", unless you count mass itself as a compressed form of energy (which it is; remember $E=m\cdot c^2)$, but if you use that quibble, then we are already beings of pure energy. :-)
Energy must be applied to particles or atoms; you must have action and reaction to get ordered and constrained rearrangement of atoms; which is what is required for any kind of machine, computation, or life. You don't get "thinking" without changing something physical.
In this case, how would be their technology?
That is a matter of fantasy, like asking about the limits of magic. It is up to the author of a fictional story, game or movie to decide how "pure energy" beings would behave and what they would build.
Maybe the galaxy itself is an intelligent living being and we are sort of its "body bacteria"?
You could write fiction that way, but IRL, no it isn't. Its parts do not interact in any way that could be described as life or thinking; the interactions we see are all far too mechanistically explainable by constant forces (mostly gravity) to be actually processing information which is a necessity for any intelligent being. It isn't pattern matching, changing to remember experience, or taking actions that would suggest any kind of goal. The Galaxy is a whole lot of molecules and rocks following gravity gradients, "Falling" basically, and that's it.
To the extent the galaxy is "intelligent", it is because of life forms within it that ARE intelligent. Those (like humans) could cooperate to become more intelligent; just like in a human brain, 100 billion neurons cooperate to make something more intelligent than any one of them. In that case, we are not analogous to bacteria, we would be more analogous to the neurons in a Galactic brain. But still not organized enough to make our Galaxy behave like an intelligent being in its own right, and due to the distances involved, that outcome is probably impossible.