Isolation and characterization of exosomes from cell culture supernatants and biological fluids from thery http://www.ncbi.nlm.nih.gov/pubmed/18228490. Théry C, Amigorena S, Raposo G, Clayton A. Isolation and characterization of exosomes from cell culture supernatants and biological fluids. Curr Protoc CellBiol. 2006 Apr;Chapter 3:Unit 3.22.
This unit describes different approaches for exosome purification from various sources, and discusses methods to evaluate the purity and homogeneity of the purified exosome preparations.
What I get to the end of ultracentrifugate is transparent pellet, (unless you aspire well pbs used as a wash can remain colored droplets) resuspended in 20 ul of PBS.
If the exosome pellet is tough to dissolve in pbs you can either let it sit for a few hours at rt or 37c then pipet up and down; or add more buffer and pipet to dissolve; or gently vortex.