We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.
In Dark Matter Still Existing, Sean Carroll of Cosmic Variance lays the topic out for readers to understand his position on this issue.
An intergalactic collision is providing astronomers with a giant payoff: the first direct evidence of the invisible material that theorists say holds galaxies together and accounts for most of the universe's mass.
CRASH COURSE. This composite image from several observatories and telescopes shows where two clusters of galaxies collided 100 million years ago. The ordinary matter, shown in pink, from the two galaxies collided, whereas the dark matter from each galaxy, shown in purple, passed straight through.
Markevitch, et al., Clowe, et al., Magellan, Univ. of Arizona, CXC, CfA, STScI, ESO WFI, NASA
What is Dark Matter? How Can We Make It in the LaboratoryConclusions
Particle physics is in the midst of a great revolution. Modern data and ideas have challenged long-held beliefs about matter, energy, space and time. Observations have confirmed that 95 percent of the universe is made of dark energy and dark matter unlike any we have seen or touched in our most advanced experiments. Theorists have found a way to reconcile gravity with quantum physics, but at the price of postulating extra dimensions beyond the familiar four dimensions of space and time. As the magnitude of the current revolution becomes apparent, the science of particle physics has a clear path forward. The new data and ideas have not only challenged the old ways of thinking, they have also pointed to the steps required to make progress. Many advances are within reach of our current program; others are close at hand. We are extraordinarily fortunate to live in a time when the great questions are yielding a whole new level of understanding. We should seize the moment and embrace the challenges.
See:What is Dark Matter/Energy?