First of all, most people don't know what the big bang theory really says. It says the universe is expanding and cooling. It says nothing about how it was created.
It is not "untestable speculation". Science is about making observations. Then proposing an explanation for those observations (hypotheses). Then testing those hypotheses. These can often be tested by other observations. It happens all of the time. In 1927 an expanding universe theory was proposed based on observed red shift of certain objects in the sky. In 1929 Hubble was able to verify the red shift and show that the further away objects were from earth the larger the amount of shift. Again, this fits with an expanding universe. In 1948 physicists speculated there would be cosmic microwave background noise signature of this expansion. In 1964 Penzias and Wilson happened across this cosmic microwave background noise (which they weren't even looking for). Well, the case for the big bang theory got a little stronger. No test tubes were harmed in the making of this observation.
What has not been found yet is anything that makes the big bang theory incompatible with what we observe.
Can we go back in time and observe the beginning of the big bang? Of course not. Can we duplicate it? No. Can we make observations? Yes Do those observations show that the observable universe is expanding over time? Yes. Do you have a better theory than the big bang theory (one that can be tested)? No.
People act like scientists just get together in a big room and make up theories and present them as fact. No, scientists make observations, create hypotheses to explain their observations and then come up with ways to test those hypotheses. And sometimes those tests simply involve other observations.
Science has it's struggles. Yes there are those who have their pet theories and are willing to fund research that is favorable to their cause. But, in the long run, scientific method takes care of that. In the end, what can be observed will trump what can be bought. The all powerful Roman Catholic Church lost the 'Earth is the center of the universe' argument because of what could be observed. Of course, we later learned that the Sun isn't the center of the universe either. But that did not change the fact that the Earth was not.
Anyone who thinks science is just another religion, doesn't understand science (or religion).