WARNING: The following contains SPOILERS for The Expanse season 5, episode 7, "Oyedeng."
Marco Inaros is the main antagonist in The Expanse season 5, but his grand scheme to "free" the Belters had a significant plot hole — one that the Amazon Prime show has finally acknowledged and, possibly, answered. Adapted from the books by James S. A. Corey, The Expanse is a futuristic sci-fi series that follows the conflict of three human factions: Earth, Mars, and the Belt. Season 5 focuses on the rise of Marco and his Free Navy, essentially a terrorist group that is able to unify the Belt (mainly through force) after attacking the Earth, causing massive devastation. In order to successfully carry out these plans, however, Marco somehow acquires very expensive Martian technology — despite him seemingly lacking the resources to do so.
In The Expanse, the Belt comprises the various colonies that exist out in space on stations and asteroids; largely a working class, the Belters are dependent on Earth for food and have essentially become an indentured servant class. The tensions between the three factions has been increasing steadily for years, and the discovery of an interstellar gateway that can allow humans to colonize hospitable planets has pushed the conflict to a breaking point.
Marco Inaros is the de facto leader of the Belters (especially after Fred Johnson died earlier in The Expanse season 5), and his attacks on Earth are, in his mind, necessary to guarantee the freedom of his people. In order to pull off his grand scheme, he required Mars technology — something the show has hinted was acquired through unscrupulous means, but never addressed fully. As a Belter, Marco would not have access to the kind of funds required to purchase this technology on the black market, even with the support of countless other Belters (and disenfranchised Martian officials looking to line their pockets). If he had that kind of resources, many of the Belt's problems could be addressed through financial means. The Expanse, season 5, episode 7, "Oyedeng" mainly focuses on Naomi, but also features a key scene between Holden and Monica that explains how Marco could be funding his operation (and why he isn't spending that money on the Belt directly): he's offering a wealthy independent party the protomolecule.
In "Oyedeng," Alex and Bobbie finally connect with Avasarala and Holden, relaying the information they have about the Martian technology getting diverted to Marco's army. When puzzling over how Marco could possibly be paying for it all — or who else would want to see Earth suffer — Monica suggests that rather than money, he must be trading something of value. She points to how Paolo Cortázar, the scientist who specialized in the protomolcule, was also abducted, and suggests that this can't be a coincidence — someone else must be working with Marco in order to regain the protomolecule and the affiliated research.
Who could be funding Marco's Free Navy in The Expanse season 5 remains a mystery, although the most likely candidate is Jules-Pierre Mao. His daughter Clarissa mentioned in the episode "Tribes" that she's not sure where her father is, and not only is Mao very wealthy, but he is personally invested in the protomolecule research — he even sacrificed his favourite daughter, Julie, for the sake of the project in season 1. Mao has shown he will stop at nothing for his research, making him the most likely character to be involved: he has the means to fund Marco, and has no qualms with extreme violence and loss of life.
Anderson Dawes is also probably involved, since he was the one who helped spring Cortázar from Tycho. Dawes has a similar motivation to Marco, so the two teaming up in order to unify the Belt makes sense — even if it means turning on former allies. At the same time, journalist Monica has someone found herself at the center of this mess, and she very well could prove to be a double agent of sorts. How the conflict with Marco will play out for the Rocinante and the rest of the main cast remains to be seen in The Expanse season 5 — and no doubt there will be more twists and turns in the final three episodes.