Complex sentence: The Cooper-Hewitt, National Design Museum, a Smithsonian Institution, is the United States' national museum of design history and contemporary design and the only museum in the U.S. whose collection is solely focused on design.
Split into simpler sentences: The Cooper-Hewitt, National Design Museum is the United States' national museum of design history and contemporary design. It is the only museum in the U.S. whose collection is solely focused on design.