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 Introduction to general relativity 
I fixed dashes in one section only, but there are more to be done; have a look at WP:DASH on the difference between emdashes, endashes and hyphens.  On matter-mass, I wasn't certain if it's a hyphen, or it it refers to matter to mass, in which case it may be an endash, but in either case, it wouldn't be an emdash.  Regards,  (Talk) "