When must one use "aller dans" instead of "aller a"?
For example, why is it used in these sentences?
"je vais dans la cuisine, je vais faire à manger", "on peut aller dans des musées"

Question

When must one use "aller dans" instead of "aller a"?
For example, why is it used in these sentences? 
"je vais dans la cuisine, je vais faire à manger",  "on peut aller dans des musées"

Answer

" aller a" could be translated by " go at" or "go to". 
example: Demain, on va à Paris  =>    We're going to Paris tomorrow
                 Nous allons aller à la fête de Martin   =>    We will go at Martin's party

and "aller dans" could be translated by "go in"
example: Je vais dans cette maison    =>   I go in this house.

Answer

"aller dans" is used to came in a place (the kitchen, the garden...).
"aller à" is for example "aller à Paris".
Hope it helps...