I understand a scientific explanation to be the simplest theory that fully explains the measured data. The measured data is: The universe has physical quantities therefore it exists. The simplest scientific theory that fits all the data we have so far as to why it exists is: It has always existed.
Even though dark energy and dark matter don't have a truly measurable physical quantity we can use other pieces of evidence, theory, and inference to strongly suggest its existence. Thus scientific explanations can be expressed in more than just physical quantities.
That's not true, where is it stated as a "law" that scientific explanations can only be expressed in terms of physical quantities? Take philosophy for example, where we can easily discuss the existence of the universe itself. It doesn't necessarily need an answer to be legitimate.
"can only be expressed in terms of physical quantities" makes no sense. Theory is tested by observation, usually after experiment. There is no "physical quantity" requirement. Poorly defining science does not prove a point.
A theory is an explanation for some observations that's expressed in terms of physical things. Newton's inverse square law of gravity refers to mass for example. Maxwell's theory refers to electric and magnetic fields.
The exact boundary of science is neither perfectly understood nor permanently fixed. Philosophy may inform it, and vice versa, as new understanding comes to light. The roots of existence are understood by nobody, and may or may not be amenable to scientific inquiry, for all we currently know.