Final answer:
We theorize about atom structure through X-ray diffraction, specifically X-ray crystallography, which provides detailed information on the size, shape, and arrangement of atoms in a crystal.
Step-by-step explanation:
If we cannot see an atom then we theorize about its structure primarily through X-ray diffraction. This method, known as X-ray crystallography, involves directing X-rays at a crystal and analyzing the diffraction pattern that results from the scattering of these rays. The patterns produced can confirm the size and shape of atoms, as well as reveal information about atomic arrangements in materials. This technique was crucial for the discovery of the double-helix structure of DNA and continues to be essential in studying the structure of complex materials like high-temperature superconductors.