It has a big mass.
Now we can see structure.
A helium nucleus is made of two protons and two neutrons.
A proton has charge +1 and a big mass.
A neutron has charge 0 and a big mass.
They both have strong interactions.
Protons and neutrons are made mostly of up-quarks down-quarks and gluons.
At the limits of experiments so far, quarks and gluons are pointline particles and are not made of anything else.
(The current limit is about 10-18 m.)
Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA firstname.lastname@example.org