I am 28 years old and just was told from my mom that my father that passed away wasn't my real father, she said my decest dad adopted me is tha possible without biological fathers signature

It may be, if your bio father couldn't be reached or found. Ask mom for a copy of the adoption papers. A court had to approve them.

Part of the issue for the OP will be they are an adult, and adult adoptions are somewhat different than adoptions of children and each state ( in the US) will have specific rules on it, even if they allow it.
If the one parent is dead, there is no signing requirement for the dead parent