The myosin filaments appear dark under microscope due to 

Myofibrils, which are threadlike structures, run lengthwise through the muscle fiber. They consist of even smaller structures, the myofilaments, or simply filaments. There are two types of myofilaments - myosin and actin filaments. Myosin filaments are thick, consisting mainly of the protein myosin. The thin actin filaments consist mostly of the protein actin; they also contain the proteins tropomyosin and the troponin complex that regulate the actin filaments interaction with myosin filaments. The filaments overlap lengthwise in the muscle fibers, producing the pattern of transverse bands or striations characteristic of striated muscle. The striations of striped muscles result from the fact that the thick and thin filaments have different refractive indices in the light microscope. Thus, myosin filaments appear dark.