Size exclusion chromatography

Some non-protein UV chromophore-bearing substance of large size was present in the sample. Whatever it is, the column separated it away from your protein, a satisfactory result.

Adam B Shapiro is probably right.

600K represents the exclusion volume for Superdex 200. So whatever you found may be larger than 600K. I suspect it's aggregate. Just for fun, you may try dissolving your sample and running the column in a buffer containing a chaotrope, such as guanidine or a detergent like SDS. This may break up the aggregate and improve yield, although any target material recovered in such buffers may lose ability to refold properly.

This kind of observation can be attributed to the presence of non-proteinaceous substances or protein aggregates that do not resolve well on SDS-PAGE. The first peak with high absorbance but no corresponding band on the gel might represent large aggregates, possibly misfolded or cross-linked forms of the protein that are resistant to SDS denaturation or poorly enter the gel matrix. Alternatively, the absorbance could be due to nucleic acids, lipids, or other UV-absorbing contaminants that co-elute at the void volume (>600 kDa region) of the Superdex 200 column. It might be helpful to check the absorbance ratio at 260/280 nm to assess nucleic acid contamination, and also try native PAGE or dynamic light scattering (DLS) to investigate the nature of the high-molecular-weight species in that first peak.