I've observed that sometimes people say something like 'domo' instead of 'arigato'. According to my observation it happens mostly when people don't really know eachother or in similar situations. Thanks.
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Here is an opinion -> rikutu.com/riyuu/008/doumo.html . According to that, "どうも" is a contraction of "どうも言えぬ" which means "beyond verbal expression." Thus, according to that (again), when someone utters "どうも" to mean "thank you," he/she uses the expressions derived from the grandiose one "I have no way to express my appreciation."– eltonjohnJul 15, 2015 at 2:28
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どうも (dōmo) by itself does not sound very formal. When followed by arigatō ("dōmo arigatō gozaimasu"), it strengthen the sentence.– narutoJul 15, 2015 at 2:30
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1But if the use of どうも to express gratitude comes from どうも言えぬ, that's purely etymology and doesn't answer the OP's question about when one word or phrase is more appropriate than the other. The question is about how they're used today, not where they come from.– user1478Jul 15, 2015 at 4:31