Since you’re seeking the indigestible, whatever it is usually has to be raw – unless it’s an onion
You’ll have heard of probiotics, because they’re very 2015, which I have identified as the year in which everyone started talking about gut health (the phrase “gut fauna” entered the vernacular a bit later). The potted version is that you want your gut bacteria to be as varied and exciting as possible, and the best way to ensure that is by eating fermented foods: yoghurts, kefir, kombucha, sauerkraut. Cultures that are alive when you consume them.
Prebiotics are different: these are sources of indigestible fibre. Roughage, broadly, is the element of a plant or legume that won’t dissolve in water, and therefore moves intact through your body, not releasing calories but doing other useful things, such as keeping you regular. There’s a sub-category within that, though, of indigestible fibres that the bacteria inside your gut can digest: almost always raw, and very often weird – dandelions, acacia gum – and this will boost your bacterial environment overall.