They could have long mastered the physics of their own bodies, eliminated cell mutations, cell aging to some extent, their bodies producing perfect new cells to replace the ones that eventually did age.
They could produce the ideal "food" or energy pill for their bodies
from common elements found throughout the universe. They
could then live VERY long lives, barring some type of accidental
death. They could be free of many of the emotions that make
us human as a race. If they were almost immortal, procreation
would become a handicap rather than a necessity.
That's my point. Sure, such aliens probably do exist. They will not have faced such things as starvation or jealousy in a long time and learned to get along with one another. But I doubt every advanced race has chosen to limit its growth in numbers. Life has a built in drive to expand and reach out. What happens when this static immortal race meets an equally advanced race of savage competitive predators?
Assuming the immortals aren't indestructible they will still die when fired at. In a universe entirely made up of peaceful alien races save for one, its that one which will expand outwards and enslave or devour its neighbours. They will be the empire builders. The main force in the universe. Pessimist that I am I can't help but think there is more than "one" fierce alien race out there.
I find the idea of peaceful species rather difficult to imagine. No such thing exists on our world and evolution is driven by survival of the fittest and most determined. From sperm, only one in millions survives. Then on to babies, perhaps one in ten youngsters reaches adulthood. Then the adults must compete for females. Species that turn their backs on competition will either devolve or simply stop progressing. Meanwhile, other more violent strains of life will continue advancing.
We humans compete from childhood onwards. For money and a nice car and a high prestige career. Everybody knows deep down that its a dog eat dog society. Everybody from the government downwards is under the thumb of somebody else. There is little real difference between the FBI and the organised crime syndicates they battle. Each is fighting over the same turf. Every nation would like to have more power than its neighbours and every person would like to rule the world.
If we travelled to another world and it was inhabited by less advanced life then somebody would find a way of making money from it.
Some Aliens might be more advanced than us in some ways and less in others. Sure. Thats what I mean by equal to us. Maybe not exactly equal but it would take centuries to discern all our comparative strengths and weaknesses during a war.
Sorry, this is really long isn't it. Got carried away.