You are just living life, enjoying your planet's culture, maybe the culture of other worlds if you have interplanetary travel capabilities. All of the sudden, no warning, no reason, half of all the people, animals turn to dust. You have no information. No explanation. They are just gone. Chaos. Grieving. No answers. Then, five years (or however they measure time) later, everyone and everything that was lost... suddenly reappears. They haven't aged, they don't know what's going on. No explanation. Ecosystems that were devastated by "the snap" are once again thrown into imbalance by the sudden re-emergence of creatures that were lost five years previous. Social upheaval. Some worlds don't survive.

And they never know what happened and why.