Which is correct?

Hi,

May I ask which sentence is correct:

Some people say that quantum mechanics is not completely understood
after its development of over 100 years and since Planck introduced the
idea that energy was quantized at the atomic level in 1900.

Some people say that quantum mechanics has not been completely
understood after its development of over 100 years and since Planck
introduced the idea that energy was quantized at the atomic level in
1900.

? You may rewrite it if you think both are incorrect or it can be
written better. Thanks.

David

.

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