As you know, Professor, in late 1984 a videogame (by David Braben and Ian Bell) was released called "Elite", which was tremendously influential not least because no-one had really seen anything like it before, and spawned a whole genre of games with cinematic spaceflight, trading, combat, upgrading of spaceships, and suchlike. It had two 90s sequels which we shall ignore, but also a recent sequel by the name of Elite:Dangerous (henceforth E:D) which seems quite jolly in spite of making the classic "we'll have Newtonian mechanics, that will be realistic... but wait, all the ships are just turrets now, we'll have to bodge some unrealistic bits in" mistake.
In E:D (and in Elite) you can scoop fuel from suns. Presumably fuel is mostly hydrogen which you fuse to make power by space magic. Hydrogen is not very convenient stuff to carry around, because even liquid hydrogen has the very low density of 70 kg/m^3. For comparison, an ordinary real-world shipping container can be filled with stuff with a density of about 620 kg/m^3. (There are some heavy-duty ones, but they don't take much more - dock cranes and ships alike can't be sensibly engineered to deal with very occasional containers which are crazily heavy). If your stuff is denser than that, you leave some of the container empty or try and find a friend who wants to ship styrofoam to the same place.
In E:D you fit your ship out by putting modules in internal compartments. Amongst other things such as optional equipment, these compartments can carry cargo. Curiously, cargo compartments, and cargo containers, are rated not by volume but by mass. A tonne of the least dense trade goods in the game fits into exactly the same space as a tonne of gold. Presumably, like real shipping containers, if you want to ship gold you leave some of the container empty - but unlike real shipping containers, you never get underweight containers even if you are shipping something that's not very dense. This is a bit odd already because in E:D your ship's mass determines thruster performance, so there would be a performance advantage to carrying light goods - or you could mix light goods and dense ones in your cargo hold (like the shipping container with the styrofoam-shipping friend), sticking to your nominal maximum weight but not leaving empty space in the way you would if you carried only dense goods - or you could fit heavier thrusters and retrofit your cargo bay to carry a lesser volume of dense goods with a higher mass.
(I'm well aware that the answer to many of these questions is "because it is a game and this makes for straightforward gameplay", yes, but this is an exercise in overanalysis here.)
This is pretty odd already - apparently the cargo bay is sized and engineered so it can only be actually filled with the least dense commodity in the game, as if real-world packing crates were engineered to be full of loaves of bread and if you wanted to ship books you could only put a dozen in each crate. However, it gets odder because these cargo compartments can also be used as a fuel store... and the space for one tonne of cargo also can accomodate one tonne of fuel.
This is really odd. If the fuel is stored as liquid hydrogen, this means all the cargo space on your ship is used to hold cargo with the density of the super-light lightest commodity in the universe - as if those packing crates were engineered to be full of packing peanuts and you could only put one book in each crate.
What if the fuel is compressed by space magic up to the density of cheese or whatever the least dense trade good in the game is? There are two problems with that. One is that when you squash up this hydrogen you will have a mass of protons and electrons all smooshed together tighter than they like, which will make them annoyed. Sometimes your ship is destroyed, and presumably then when that happens all of the smooshed-up hydrogen will unsmoosh in a tremendous hurry, pushing bits of your ship away in their eagerness to escape, and since weapon ranges dictate your assiliant is probably within a few kilometres the aftereffects will make them rather annoyed for a very brief period of time and then exceptionally dead. (This is not perhaps a problem from your point of view, but first of all it doesn't happen and secondly there are more assiliants than would seem plausible if it did).
The second is the internal volume occupied by these cargo or fuel compartments. If they really are so big as to carry their full rated weight of unsmooshed liquid hydrogen at 70kg/m^3, they're pretty big - on a Type-9 Heavy, one of the more capable freight ships, the maximum capacity of 596 tonnes of fuel will occupy circa 8500 m^3. The ship's cuboid bounding box is about 450000m^3 and if we guess it occupies about half of that then the cargo volume is about 1/3 of the total. That seems fine, but if cargo and fuel were stored at a less expansive 350 kg/m^3 (much less than a real world shipping container) then only 1/15 of this dedicated freight hauler's internal volume is actually used for hauling freight. (Figures for other cargo-oriented ships are not radically different).
Well, maybe it is, we might say. Spaceflight is difficult, especially with all that violating causality the space magic FTL drive lets you do, and perhaps the rest of the ship is chockfull of heavy equipment and only a tiny bit is left over for cargo? Alas, no, it isn't - not dense equipment, anyway. That same ship with all the standard equipment that doesn't go in one of those cargo/fuel slots - indeed, upgraded with a full complement of guns - weighs 1246 tonnes, or in other words 5.5 kg/m^3. This is quite light - for example, it is about 500 times less dense than aluminium, or 12 times less dense than the liquid hydrogen which is so inconvenient to store because it is so very not-dense. It's not even 5 times as dense as Earth's atmosphere; you can't land on planets with atmospheres, and now we know why - in a high wind your ship might very well blow away.
It is very hard to imagine that the rest of the ship is full of anything, and if we start supposing the cargo is plausibly dense there's even more nothing to fill up. So what's the point of this, besides that the spaceships of E:D appear to be gossamer-thin constructions for people who think space is so awesome they like to have quite a lot of it on the inside in spite of the plentiful supply on the outside?
Let us return to 1984. One of the peculiarities of the original Elite universe was that it was full of pirates. Wherever you went there were pirates aplenty, who would attack you with complete disregard for personal safety, in spite of the fact that you destroyed half a dozen of them every time you went anywhere and almost never saw any innocent traders so necessarily they were almost never successful in any act of piracy. Furthermore, I do mean wherever you went. You could be banished by a hyperspace misjump lightyears from the nearest star, and who would turn up after a while? Pirates.
Any reasonable estimate of the universe's minimum pirate density, combined with the relative dimensions of space, pirates, stars, and planets (the latter two are really rather small compared to the universe we know), leads one to conclude that it is at least 99% comprised of pirates. This may explain their suicidal behaviour - what does it matter if the hive loses a thousand drones to bring you down when there are untold trillions packing every corner of the void?
In the middle of the twentieth century, there came a point where it was generally accepted that the universe was expanding, but the Big Bang was only one of two theories explaining this state of affairs. The other was the "steady state" theory, which suggested that as the universe expanded, hydrogen atoms spontaneously popped into being and gradually ran into each other until they had enough friends to make a star, which would fill in the gap left by some existing stars moving away from each other.
Of course the steady state theory is no longer taken seriously, but there is no particular reason why Braben and Bell, in 1984, couldn't decide that the fictional universe of their game followed this cosmology. Clearly they needed an explanation for the endless supply of pirates, and decided simply that pirates were entirely made of hydrogen, this being how the Elite universe replenished its supply of suns. This is perhaps why in the later BBC Master version of Elite the stars are .001% further apart, although on the 320x256 screens of the era it cannot easily be seen.
This did, naturally, raise the question as to whether the innocent trade ships, police Vipers, and indeed the player themself were humans in steel hulls, or also made of the same gossamer hydrogen star-stuff. That question, I'm pleased to report, has now been answered; every ship in the Elite universe is a wisp of hydrogen, born from the interstellar gulf and someday to grace it as a new star.