How is ~73% of the universe made up of Dark Energy if energy doesn't take up physical space?
Energy is a propery of a system. E.g. potential energy of a body in a gravitational well of another body, or kinetic energy of a body w/r to some reference frame.
That's why you can define energy (density) of the universe - it's a system that has that property.
'Dark energy' is a place holder name for something we don't know which appears to be causing accelerating expansion of space.
We don't know what dark energy actually is, but it certainly exists in physical space and not somewhere else.
It isn't baryonic matter of course, atoms, but then other more usual forms of energy are not either.
Does light or electricity occupy physical space?
Force carriers, called bosons, are massless and do not occupy space, only massive particles, called fermions, occupy space so there is no limit on the number of bosons that can occupy any given space.
Not all bosons are massless. Mesons are bosons and have a mass. But, of course, all bosons are volume less, since they are not subject to Pauli's dxclusion principle.
I stand corrected. The meson is indeed a boson with mass. Upon further review I noted this discussion at https://www.fnal.gov/pub/today/archive/archive_2013/today13-02-15_NutshellReadMore.html Fermi lab
" The quarks, leptons and bosons of the Standard Model are point-like particles. Every other subatomic particle you've heard of is an extended particle. The most familiar are the protons and neutrons that make up the nucleus of an atom, but there are many others—pions, kaons, Lambda particles, omegas and lots more. The defining feature of these kinds of particles is that they have a reasonably measurable size (which happens to be about the size of a proton).
It is further stated
" Point particles are much more bizarre and are sometimes said to have zero size. This statement has raised more than one eyebrow. How can something have no size at all? And if it has mass, does the zero size mean it has infinite density? (And by the way, as you read on, you'll see the answer to that last one is no.) You begin to see why some people are skeptical when a scientist says a particle is point-like. ...While the quarks, leptons and force-causing bosons of the Standard Model are all currently treated as point-like, there is no guarantee that this will always be true. It may be that as we probe to smaller and smaller sizes, we will eventually find that the particles we thought were point-like are actually extended particles with smaller things inside them. However, because the core particle is surrounded by this extended cloud, determining whether the core is point-like or extended is a real challenge."
For futher discussion that may be of interest see;http://journals.aps.org/prd/abstract/10.1103/PhysRevD.86.034003, Size of the σ meson and its nature.
But if today's elementary particles are extended objects with smaller things inside, what are those things--pointless or extended? Ultimately either we must accept point particles or have extended objects which are not rigid. If elementary particles are extended and rigid, we have another problem: any force applied on one side cannot be communicated immediately to the other side (because no communication can take place faster than velocity of light) and so the whole particle cannot move together. In other words it cannot be rigid.
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