“What is “space-time”? Space is a word used for “where there isn’t anything.” Time is the measurement of motion. (Thomas of Aquinas.) Putting them together with a hyphen produces what?”
Not made up, but conjectured by math (Einstein) and proved every day when one uses GPS.
It makes no logical sense, but as have many things in physics, when the math says it exists, it exists. Sorry, but that’s life in this universe. God is a mathematician.
Space-Time is properly hyphenated because it links two things - space and time.
Let’s consider space. Space is defined as a place for matter (room for it to exist)
If you have 1 dimension (a line) the volume is zero. 2 dimensions you have a plane (which no depth, volume = 0), once you have 3 dimensions, you have infinite volume.
Time is likewise 3 in nature. For change to occur, one must have a past, present, and if time is to continue, a future.
Finally, the 3 unexcited (at rest) states of matter are liquid, solid, gas.
Our universe is Tri-unity in nature - that is, 3 threes. If you look at nature, you will continue to find the mystery of 3 - proton, electron, neuron - Man, plants, animals, etc.