Let's posit that:
The universe must reach a certain age for complex life to develop since generations of stars must live and die for heavy elements to accumulate in planetary crusts. Once the universe reaches that age, many civilizations will spring up around the same time (within a billion years or so.)
Even if 1/1000 alien civilizations are expansionary, it will be the expansionary "grabby" aliens that come to dominate the universe. By the same logic, the fastest spreading civilizations will dominate, perhaps spreading at 90% of lightspeed. This means that there will be little warning that they are coming.
After some time, aliens will dominate most "livable space" so no new civilizations will arise. This means, there is only a small chance that any alien civilization can see others spreading: either they are outside their light cone or the time window of a mostly "empty" universe has closed and no new civilizations form.
This Fermi paradox explanation is from Robin Hanson. Now let's add Nick Bostrom:
What will aliens do with all domains they control? They are almost certainly machine-based hyper-intelligent entities with massive computing power. They may decide to spend their time writing poetry or pondering the heat death of the universe, but there is a much more pressing concern: meeting other hyper-advanced aliens, with completely unpredictable, and possibly fatal results. Remember that they can't see the other aliens, since both are spreading at 90% light speed, from approximately the same starting period, and both aliens will be the type that aggressively spread. What can the aliens do to mitigate this risk?
One solution would be to run simulations of the universe and evolve other aliens until they reach hyper-advanced status. The aliens can research the simulation to model how a meeting would go, or perhaps learn some technologies from them. The more such civilizations they model, the more insights they can gain: remember that they have near-limitless computing power.
So that's my prediction: out of all civilizations, the vast majority are likely to be simulations for the purpose of making first-contact scenarios safer.