a galaxy would need 2.55 times (or is that percent?) more dark matter

That is a factor of 2.55, ie an increase of 155%.

Would the (hypothetical) physics of all the galaxies being 23% closer and 23% bigger have any bearing on individual galaxy rotational curve and velocity?

You could hypothetically imagine a galaxy from which all the dark matter is suddenly removed. The outer parts of this galaxy, deprived of the attraction of Dark Matter, would fly away into intergalactic space, never to return. The inner parts of the galaxy would fly out further, only to return, in highly elliptical orbits.

You could (hypothetically speaking) imagine that when you removed all the dark matter, you simultaneously increased the mass of all the visible matter by 23%, and shrunk the size of the galaxy by 23%, without changing the angular velocity of the stars. This would boost the gravitational attraction of the galaxy, enabling it to hold onto the outer stars. However, the galaxy would look quite different than it does now, with stars that are much brighter. The rotation curve of this new galaxy would be quite unlike what we see now, with the outer stars moving more slowly than the inner stars (after it settled down from this major disruption).

dispense with the idea of dark energy

I have ignored dark energy here. When you are talking about the scale of individual galaxies (or even clusters of galaxies), you can ignore dark energy in the current universe.

Some physicists have suggested that if Dark Energy is growing stronger over time, that clusters of galaxies could be torn apart, as could our galaxy, our solar system, the Earth, and even our atoms, in an event nicknamed the "

Big Rip".

consider a non-expanding universe based purely on inter galactic gravitational and centrifugal forces?

Einstein did initially consider the universe to be non-expanding.

However, it was discovered that this is an unlikely scenario, requiring a big coincidence in certain values in the universe.

More likely is to find a universe that is expanding or contracting; a non-expanding universe is unstable, and will collapse.

As you suggest, a rotating universe may well be able to sustain itself against gravitational collapse into a "big crunch".

I'm not at-all trained in maths, but my analysis is that all formulas and geometry are actually but an expression of percentages in relation to each other... ?

Scientists, mathematicians and Engineers do like "invariants" - things that are true, regardless of the size. That way you only need to remember one fact, instead of different facts for different sizes.

So the angles of an equilateral triangle are all 60°.

If you increase the length by 23%, the angles are still all 60°. This is an invariant*.

However, there are some things that change with scale in a defined way:

- So if you increase the radius of a sphere by 23%,

- the surface area of the sphere will increase by 51% = a factor of 1.51 = (1+23%)
^{2}

- and the volume will increase by 86% = a factor of 1.86 = (1+23%)
^{3}

- In this case, the "unchanged facts" are that the area increases as the square of the radius,
- and the volume increases as the cube of the radius;
- these invariants are expressed as a formula.

Note that just adding 23%+23% does

*not* produce 51%, and adding 23%+23% +23% does

*not* produce 86%. You can't just add and subtract, but for cosmology you must also multiply, take exponentials - and that is not even taking into account Einsteins relativity!

*I know I am ignoring non-Euclidean spacetime, but let's get beyond addition & subtraction of percentages, first...