How can dark energy, a so called weak force can overcome the effect of nuclear force, strongest force known till date?

The answer to this question lies almost in the question itself. Nuclear force is the strongest force till date, an event such as tearing of atoms (if it occurs at all, as they are only speculations, neither theories, nor even hypotheses) will occur only in an era where dark energy dominates the universe, then of course, the force (it will act like a negative pressure force) of dark energy will be much stronger than all other forces. There is no reason to believe that the relative strength of the forces will always remain the same. Actually as a matter of fact, the relative strength of gravitational force (however, according to GR, its not at all a force and is rather the effect of the curvature of space-time continuum only, but let's say we are talking about the force-like effect of this curvature) as compared to all the other three forces have known to change during the history of the universe (it happened during the period when universe entered into the matter-dominated era from radiation-dominated era and this incident is closely connected with the formation of the large scale structure of the universe).

Don't try to think in terms of physical force or something like that. Whenever you seriously study some quantum mechanics, you will know that it is not quite scientific (of course referring to the realm of present day physics) to talk about "force" physically. It is much easier and much less confusing to talk about these things mathematically without worrying too much about developing a physical feel about it for example, many people, even HS level text books refer to the concept of electron spins by an analogy with a spinning top, if you believe me these two concepts have nothing in common. The spin angular momentum of an electron (or any fundamental particle) has no classical analogue at all.

Cosmological constant (dark energy) is a constant energy background that fills up universe and exerts negative pressure is equal to cosmological constant whereas pressure is positive for radiation and ordinary matter.

In early stages of universe, radiation was dominant form of the energy but as universe cooled down, energy density due to radiation decreased rapidly and then came an era when matter formed a dominant part of the universe but as the universe expanded further, matter started losing its dominance and then came an era when dark energy started dominating. Energy density due to dark energy is constant and very small and hence in early universe, it was too small as compared to radiation and matter density but as universe expanded and cooled, radiation and matter density decreased to such extent that dark energy has started dominating now

Also electromagnetic forces are strong, but overall matter on the large scale is neutral and hence this force is also irrelevant as far cosmological phenomenon are concerned.

Gravity becomes important when we consider collective action of a lot of matter clumped together like galaxy stars, etc. Dark energy is much weaker than all these but it fills the space all over, so if we want to see its effect, we have to talk on large length scales where collective effect of dark energy will be greater than that of gravity.

This is the reason why galaxies move away from each other at accelerated ever-increasing rate because on cosmological distance scale, collective effect of dark energy dominates all other interactions.