* No known substance can dissolve everything: All known substances have limitations in what they can dissolve. Even the most powerful acids, like hydrofluoric acid, cannot dissolve everything. There would always be something that resists its dissolving power.
* Dissolving involves chemical reactions: Dissolution is a chemical process where the bonds between molecules are broken and new ones are formed. There would need to be a chemical reaction that could occur with every single substance in the universe.
* The container problem: What would you store this "universal solvent" in? If it dissolves everything, it would also dissolve the container it's stored in! This creates a practical impossibility.
In short, the claim of a "universal solvent" is a scientific impossibility due to the fundamental nature of chemistry and matter.
This claim is likely a science fiction trope that has been used in stories for dramatic effect.
