Indeed. When I'm debugging a program and feed it new data and something completely unexpected (and obviously off-the-wall) comes out the other side, I always ask myself "Wait, what am I assuming?".
This is what drives me absolutely batshit about modern cosmology:
1) The speed of light is constant, everywhere and everywhen.
2) The gravitational constant is the same, everywhere and everywhen.
3) The shape of space is uniformly flat, everywhere and everywhen.
4) Please don't get me started about standard candles.
5) Or cosmological inflation.
6) Or the (luminiferous) aether. Sorry, the Higgs field/particle/whatever.
Therefore: Dark Matter!!! Dark Energy!!! QED!
Picture a boulder embedded on a steep hillside. We say it wants to roll downhill, but it really wants to fall to the center of the earth, it's just the shape of its local space that constrains it to roll downhill. Now picture a pebble sitting on top of the boulder. It's attracted to the center of the earth, but also (yes, yes, weakly) gravitationally attracted to the center of the boulder. It wants to roll down (two!) hills. Actually, it's also attracted to the Sun, and Jupiter, and the Milky Way black hole, and Andromeda; a zillion other force vectors get simplifyingly assumed away. And we're in the position of being an ant sitting on top of the pebble, making local observations and universal assumptions.
To my way of thinking, modern cosmology has posited a spherical cow of uniform density, and is baffled about where milk comes from.