definition of numbers

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do most people understand the definition of certain numbers such as (the simplest one) of one?

As in, if you have one marble, it seems clear what that means.

But what about one solar system? It becomes less clear?

Then one galaxy, even more difficult to define which areas of the universe this applies to.

Then one universe, which even astronomers disagree on the definition.

I don't think it's the numbers that are the problem, it's the definition of what the thing is that the numbers are being applied to.

When is a marble not a marble? They're usually made of glass, but I guess they'd count if they were made of any other hard substance. But how soft can a marble be before it ceases to be a marble? According to the World Marbles Federation,
2.3 The marble is a sport equipment from a solid, non-metallic material with unchanging shape and properties. It should be no greater than 16.5 mm in diameter. Players are obligated to present their marbles to the referee at his request for a check.
http://www.worldmarblesfederation.com/marbles-rules.php

But:
1. I doubt there's such a thing as a solid of unchanging shape and properties. The changes may be extremely small, but they do exist. How much change is allowed?
2. Those rules don't seem to mention how near to a perfect sphere a marble must be.
3. How do we define "sport equipment?"
4. Why must we take the word of the Federation as the definitive answer to what a marble is?

But maybe there is also a problem with numbers. Nearly everybody knows exactly what they are, but I've yet to see satisfactory definitions of "one," "two," etc.

All your points are solid.


However, I also feel like certain classes of things (such as universe) we aren't sure of what the actual definition is.

It will all make sense once you study and learn Set Theory.

Sure, we can't be sure exactly what a marble is, and many other things can't be defined perfectly either. But I'm fairly happy with defining the universe as everything - all space, time, matter, energy.

since I as a layman even can't properly define the physical manifestation of time and matter and space and energy, I would have to disagree.

But I presume you have a fair grasp of what "everything" means? I don't understand the exact nature of all the individual parts of everything, but if it's a thing at all, then it's a part of everything. Really it's just the definition of "every." That is, without exception. I don't see anything to disagree about - I simply see the universe as everything. That's just my personal convention and definition of the word. Others may have their own conventions, but nobody can prove or disprove a convention, so you may define things differently to me.