A lot of things in the universe can be measured, but people want to know why they are that size. We know the mass of a quark and the charge on an electron. These are constants. It turns out that these numbers have to be exactly what they are, because if they were different we would not be here. You, me and the physicists, we’re part of the universe. We have to be here to make physics, so physics has to describe a universe where there can be people. Gravity is another problem for physics - because it is everywhere and acts on everything, including space itself. It’s different from other forces like electricity or radiation, because you can’t stop it or turn it off.
Modern physicists still look mostly at things we can’t see. (They think gravity might happen because of something no-one has ever detected called the Higg’s boson.) Either very small things in quantum physics or very big things like galaxies. Putting them together is the main problem of modern physics. The universe and space and time described by Einstein and the fuzzy fast-moving little sub-atomic particles and small things that might make them up. If you want to know how the universe began - with a tiny size but very big mass, then you need a theory that fits both together. At the moment, the theory suggests that the things we can see – stars and planets etc. make up only 5% of the universe. The rest is 25% “dark matter” and 70% “dark energy”.