dark energy really as strong as physicists state?

One possibility for dark energy is that it is due to the negative pressure generated by vacuum energy. The idea is that the our current vacuum is what is called a false vacuum which is a vacuum that is not at the lowest energy level possible for it. Since positive pressure is always exerted towards our current vacuum, negative pressure would be generated if our vacuum exerted pressure away from itself toward a lower energy vacuum state. When you plunk negative pressure into the mass-energy part of Einstein's General relativity field equations, you get an expansion of the metric rather than a contraction of the metric as would be expected by positive mass/energy. There is a lot of reason to believe that after the big bang the inflationary period of the universe was due to negative pressure.

The problem is that there is no way to tell for sure whether our current vacuum energy is in it's ground state or is at some higher level (false vacuum). There are some theoretical reasons however that would indicate that it is possible we are in a false vacuum.

The idea of a false vacuum may seem very strange, but NASA will be using it to try to demonstrate the plausibility of the Alcubierre drive (you can create a local equivalent of a false vacuum using the Casimir effect)

"Dark energy really as strong as physicists state?"

Sure. It is stronger than gravity. But since we know now that gravity is NOT a force, Dark Energy is no sort of energy... just a description of how *now* moves away from the past.

"I was watching alien encounters and it said advance aliens might know how to use dark energy to destroy us."

Any aliens we might encounter, will know better than we what Dark Energy is, and could give a flip about us, much less killing us with DE.

"So cant we just use energy to cancel out dark energy?"

No, since that only serves to aid gravitation. Which is nothing.

Remember, "Dark" just means "unknown". In the case of "Dark Energy" is is just another name for the Cosmological Constant (that doesn't have to be constant).

The energy is "dark" only because we've no clue what it is. We know only what it does. And that's expand the universe at an accelerated rate.

From your "So cant we just use energy to cancel out dark energy?" you seem to imply that dark energy is some sort of anti-energy, like anti-matter is to matter. That's not likely as the dark energy is pushing the universe into accelerated expansion, just like ordinary energy would. No, dark energy is ordinary energy in that respect; it's just dark because we don't know where it comes from. [See source.]

Some day we hope to shed light on the dark energy by finding out what it is; so it won't be dark anymore.

It's important that you know that dark energy isn't anti-energy, so it doesn't just cancel out when in the presence of regular energy. Dark energy Is the stuff that is stretching the Universe... so if alien's come after us using dark energy weapons, we're pretty well screwed

One day dark energey broke all atom of universe