<blockquote> Why is it believed that planets, stars, and galaxies have dense cores, and not vacuum/void centers?</blockquote>
As per the subject of this thread, there ain't going to be no danged polar openings that will amount to much on a solid core planet. And so, back to:
F = G X m1 X m2 / r^2...
...which might answer the question in the quote. It is because Newton essentially used his cannonballs to weigh all the celestial spheres. Add to that, by him guessing that gravity is merely some magical characteristic of all matter, everything we now take for granted as the nature, structure (and mathematically projecting, by way of this formula), even our current ideas of the origins of the seen and unseen universe, goes back to this double-edged single assumption. Everything science proclaims about the macrocosm has erupted from these assumptions about mass and gravity. I like to call it, "mathturbation." (And I don't lisp.)
We have, after four hundred years, seen the whole of physics grow out of the values projected from the trajectories of cannonballs. Yes indeed, we sure can put cannonball-like objects into orbit, all day long. This still doesn't mean that cannonballs have gravity, or that Earth has a solid nickel/iron core, pressed in upon so hard by way of the back-tracking of the inverse square rule (preset in that equation) of all the mass of solid Earth, creating so much pressure at the center that even though the nickel and iron which is beyond its melting temperature, those metals have been forced to crystallize into a solid. And even though iron loses its ability to maintain a magnetic field at somewhere around 800F, we are to believe this imagined solid hot core supports the geomagnetism of this sphere.
Then we might see some doublethink, when it is seen that at the center of mass of a solid sphere, all the mass of the sphere is outward from this center. If mass is what makes gravity, it should all be pulling away, omnidirectionally, from the center. If mass has the gravity, what accounts for the pressures needed to crystallize the core? Only in science can we have it both ways, eh?
Earth and Moon ring like bells, seismically. Hollow spheres can ring. Solid spheres will thud.
Science tells us, "The core is solid because the P waves can't go through the solid core." That's what we're told, all right. They say this proves it. I bet the seismic waves would do better traversing a solid core than a hollow core. What do you think? Well, unless we go into the polar openings, we won't know.
It can be shown that fluid vortex motions develop E- in their centripetal whorls. If we've blown all the math by way of that first equation of cosmology and cosmogony, and we're calculating masses far in excess of the reality for all the celestial spheres and their spiraling conglomerations (the galaxies, which look as suction spiral motions to my eyes), we may have to start over at nearly the beginning. Doing so, how many questions might be more easily answered about the nature of damned nearly everything?
I see hollow spheres fitting right in with the Electric Universe. Solid, well, not so much.