<p>I don't think the problem is solvable just by closing more questions or downvoting. I think all users start at a point where they just ask questions, then some of them become knowledgeable enough to start answering questions, and get hooked in the process, so the number of answers/time increases. Then, as in all human endeavours, they reach a point where they start losing interest and/or they have less time to spend in this activity, thus the rate of answers decreases (save for a few exceptions). Since new askers keep coming, but just a fraction of the askers become answerers, I think the only solution is to find a way to increase the number of answerers (maybe encourage short answers, by upvoting them) or to just accept that the gap will increase. </p>

<p>As Nick Cox correctly noted when commenting on the OP, "[..](downvoting, cleaning up, etc.) is as exciting as tidying up an untidy cupboard/closet when the alternative is watching one of your favourite movies or TV shows". Thus if we can't manage to increase the number of answerers, <em>a fortiori</em> I don't think we can manage to increase the number of downvoters/closers/etc. It's true that the last two activities take less time than writing good answers, but it's also true that they're harder to pitch to users.</p>

<p>Another possibility is that the gap is mostly due to an increase in low quality questions: Martijn Weterings' answer provides (now stronger) evidence in that sense. However, given that it takes little time to write crappy questions, and that there is a surge of interest for some topics related to Statistics &amp; Machine Learning currently, I don't think the rate of crappy questions will go down easily.</p>

<p><strong>UPDATE</strong>: with <a href="http://data.stackexchange.com/stats/query/872982/can-i-find-the-number-of-total-questions-and-unanswered-questions#resultSets" rel="nofollow noreferrer">this query</a> I retrieved the number of total &amp; unanswered questions per month, for each of the 10 tags with more unanswered questions (today). </p>

<p><a href="https://i.stack.imgur.com/XimVu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XimVu.png" alt="enter image description here"></a></p>

<p>The result suggests that <code>r</code>, <code>regression</code>, <code>machine-learning</code> and <code>time-series</code> are the main drivers behind the growing chasm. Thus, concentrating the VTC &amp; "recruit new answerers" efforts on questions with one or more of these tags should provide the most benefits. I remain skeptical of a sustained solution, i.e., I predict that answerers/closers will get bored of engaging in these endeavours soon before the question rate will drop. But let's see...</p>
