Learn Swedish: Vi måste förstå molekylen. Hypotesen är nämligen komplicerad. - We must understand the molecule. The hypothesis is complicated, you see.

Even though nämligen explains the reason for the previous sentence, it is a sentence adverb, not a conjunction like eftersom (because). In a Swedish main clause, sentence adverbs must be placed right after the finite verb to obey the V2 rule. You can think of it as sharing the exact same grammatical slot as inte or alltid.

The translation ends with 'you see', but doesn't nämligen usually mean 'namely'?

Yes, it often translates to 'namely' when you are specifying a detail. But when used as an explanatory sentence adverb like it is here, it provides the 'why' for the statement right before it. English doesn't have a direct equivalent that sits in the middle of a sentence like this, so we often capture the feeling by adding 'you see' at the end or starting the sentence with 'Because'.

Why doesn't the adjective komplicerad have a -t at the end here?

Adjectives in Swedish must agree with the gender of the noun they describe. The word for hypothesis is en hypotes, which is an en-word. Because it is an en-word, the adjective komplicerad stays in its base form without the extra -t that ett-words would require.

Both molekylen and hypotesen take the -en ending. Is it common for scientific terms to be en-words?