I wasn't thinking of matter as such there, more as an 'ethereal observer' at any point in the expanse between galaxies. And I went from the definition I understood comoving systems to have reading about it.

"While general relativity allows one to formulate the laws of physics using arbitrary coordinates, some coordinate choices are more natural (e.g. they are easier to work with). Comoving coordinates are an example of such a natural coordinate choice. They assign constant spatial coordinate values to observers who perceive the universe as isotropic. Such observers are called "comoving" observers because they move along with the Hubble flow.

A comoving observer is the only observer that will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

Most large lumps of matter, such as galaxies, are nearly comoving, i.e., their peculiar velocities (due to gravitational attraction) are low.

The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a comoving observer and is a measure of cosmological time. The comoving spatial coordinates tell us where an event occurs while cosmological time tells us when an event occurs. Together, they form a complete coordinate system, giving us both the location and time of an event."

And that's why I couldn't put it in 'practice' thinking of it. Could I assume that it is a way to negate the expansion mathematically? "Two objects are comoving if the distance between them is increasing at the rate of the Hubble expansion times the distance." That one makes perfect sense to me if I assume that.

This one was a little harder

"The velocity of two objects relative to one another is defined as the vector difference of their velocities relative to the coordinate system;"

A vector is a direction and a magnitude(speed) right? Then you write "so they have zero velocity relative to one another, even though the distance between them is increasing." Can I assume that to mean, if I now get it right, to mean that if there is no vector difference found/measured between them, they can be described as comoving? Which I then translate to having the same speed and direction, ignoring expansion?

I think I will have to read this after sleeping actually

, it's a different approach to SpaceTime than what I'm used too. It's not that you are unclear, blame it on my thickheadedness