Question

Are dark bands formed because of the accumulation of myofilaments there?

Answer 1

In a myofibril there exists many interwoven actin and myosin filaments. In a sarcomere there exists alternating I and A bands with varying electron density under EM.

In a dark band (A band), thick (mainly myosin), and thin (mainly actin) filaments overlap.

In a light band (I band), it is lighter because it contains only thin, actin filaments.

When your muscles contract, and myosin undergoes conformational change due to its ATPase activity, the active myosin "pull" the actins from the I bands through the A band.