We have recently developed a new system of refractive-index matched nanoparticles. This system has advantages over a extensively used one of polymer latexes; ours 1) can be prepared directly in the solvent of interest and 2) consist of a single polymer molecule in a single solvent. The particles are very interesting optically, as they are essentially transparent at all concentrations; our studies have been up to 50% by volume. However, more importantly for physical scientists, they particle interactions are minimized, making them, as far as is experimentally possible, equivalent to hard spheres. The next challenge in establishing them as a model system is verifying that the nanoparticles interact as hard spheres. Spin-echo small-angle neutron scattering (SESANS) is a uniquely suited tool to characterise the interparticle interactions.