What is mass?! I knew it wasn't particles, but....
99% of people don't know what mass is.
Mass is not the amount of some particles or matter.
Mass is energy limited by the system.
99% of mass is contained in the nuclei of atoms, which are made up of protons and neutrons, which are made up of quarks.
So 99% of mass is contained in quarks?
This is shocking, but quarks only account for 1% of mass.
But then where is the rest of the mass?
99% of mass is the kinetic energy of the gluon field, which holds 3 quarks together, creating the strong interaction.
It is important to note that 1% is also energy, but the energy of the Higgs field.
Mass is measured not in kg, but in megaelectronvolts.
So it is appropriate to say that we are made up of energy, and Mass is energy limited by the system.
And also, in GTR, mass is relative and in general it is not mass, but momentum, but no particles or kg.