I was wondering if you could convert gold into silver. Like, if you are running low on silver, but have extra gold, could you convert the gold into silver?

There is not a way to convert it sadly. However you can convert it in a roundabout way. You can purchase items from the premium shop and then sell them to users. Though it may be easier to post in the item sales saying that you are willing to buy premium shop items for users. That way you are buying only what you know others need. I've done this before- it seems mutagen and energy drinks can be a hot item, especially duirng events (founder cells and ephemere cocoons also become very wanted during events). 

Nice avatar by the way. I'm glad you like my Loki skin :)

No problem. 

I think the reasoning behind that was based on game mechanics and economics. If the game itself had a set currency conversion it would limit the economy to a degree. By having no set value it really allows things to fluctuate more I think. For instance if you went to the premium shop and bought a bunch of energy drinks they would be worth a decent amount. But if you were to hold on to them, or wait and buy them during an event....  Now that would be worth a lot more!!!