The top copy of a pile of exercise books had a teacher’s name spelt wrongly, “Miss Flanders” had become “Miss Flounders.” Trying to cover a Year 7 science lesson, I might have renamed myself “Mr Flounders,” for flounder, I did.

It was a lesson on non-contact forces: gravity, static electricity, and magnetism. Units of measurement created problems. The idea of weight being measured in Newtons and mass being measured in kilograms was a concept as difficult for me to grasp as it was for the twelve year olds in the room.

We discussed how gravity held the Moon in orbit around the Earth and how gravity held the planets in orbit around the sun. We read that the greater the mass the greater was the gravitational pull and how that gravitational pull lessened as the distance between objects became greater.

Not being a science teacher and it being a Year 7 class, I adhered strictly to the exact wording of the textbook, but at the back of my mind there was a nagging doubt about what I was teaching. Would there be some future moment when students her pursued the subject at a deeper level realised that the lesson had not corresponded with the reality of the gravity that held the universe together.

My understanding is that if the universe is held together by gravity, then there must be something out there that we cannot see, because there is not sufficient mass in what of the universe we we can see to hold things together.

If the only gravitational pull holding galaxies together is that which is exerted by the matter that can be seen, then the velocity at which some galaxies turn should cause stars and planets to go shooting off into space.

For our theories of the universe to work, something more must be present in the galaxies. There must be something that we cannot see exerting a gravitational pull. There must be some form of dark matter, some sort of invisible stuff without which our universe just would not be the way it is.

For decades, the physicists’ answer to the riddle of why the universe does not fly apart has been WIMPs – weakly interacting massive particles – yet the search for such dark matter has continued to be unsuccessful, efforts to detect the WIMPs have so far been fruitless.

Eventually, however, the source of the necessary gravity will be identified, and the school textbooks will be revised. Perhaps a time will come when the current lessons will be regarded as primitive as models of the solar system with the Earth at the centre.