In the example: 'Il y a un expert dans tel domaine veut partager sa connaissance avec des gens.'
Why is tel used? I thought tel means 'such', so why in the above sentence is 'chaque' not used instead?
First of all, this is not a correct sentence. The sentence was probably:
Il y a un expert dans tel domaine qui veut partager sa connaissance avec des gens.
You are not wrong in translating "tel" by "such", as it's indead the case in "tel que", which translate as "such as". But in this context, the correct translation for "tel" is "that" ("such" is ok but not as good IMO).
Your sentence translates to:
There is an expert in that domain who wants to [...]
There is an expert in such domain who wants to [...]
Here are some tranlations of tel:
Une chose telle que celle-ci -> A thing such as this one
Tel père, tel fils -> Like father, Like son
Tel l'oiseau qui vole, ... -> Like the flying bird, ... Je ressentais un tel dédain... -> I felt such disdain