Question: My question is should I use *was* or *were* in the following sentence:

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> ...this is one of the messages, which *was*/*were* sent in the space 37 years ago. 
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Which one I should use?

Answer: 
It depends on which noun or nouns the phrase "went into space" is modifying.
If a number of messages went into space and this is one of them then the verb belongs to 'messages' and is 'were'. If there were a number of messages and this one alone went into space, then the verb belongs to 'one' and is 'was'. Either your comma is there by mistake, in which case it would be 'messages which were' or the comma is there on purpose which would make it 'one of the messages, which was'. The latter sounds a bit awkward so if that were the case I'd rephrase.
I would normally use the expression **“one of”** with a plural noun. 

Reason:

It could be a group of people or things. *“One of”* informs the listener that you are talking about one member *of the group.* 

Since you are talking about **one message of the many messages**, you use a singular verb after the plural noun.

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> "...this is one of the messages, which *was* sent in the space 37 years ago".
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